May as well chime in. Contrary to what a lot of other people have done (chime in with their opinion consisting of a few sentences) I feel like I should make a longer post; not because it is superior to briefly posting your thoughts - what the goal of this topic is - but because there are already 30+ pages of this and throwing in my 2-3 line thoughts are probably overkill (and may not even be read). I will try to summarize what a lot of people have already said and interject my own thoughts as well.
Offensive pokemon vs. Defensive pokemon
First of all, the most common complaint is the amount of threats and lack of defensive options (or defensive 'threats' even) in today's metagame. I totally agree with this. This doesn't mean that we like to play 300-turn games back-to-back and solely rely on spamming status and hazards, it simply means that it creates a somewhat boring onesided game that doesn't reward switching. Additionally, the lack of defensive pokemon puts a strain on teambuilding, there are simply so many threats to keep track of that eventually you'll find yourself helpless against something like a sub/focus blast/roost Hydreigon, no matter how good your team is. You may not see that particular set in your next 100, 200 or even 300 games but due to the sheer amount of threats you may encounter a scenario like this in a large percentage of your games where you are swept by a rare moveset/pokemon.
Unlike in for example gen 3, you cannot say 'alright, Skarmory and Blissey... lets add Claydol too, now I can wall a lot of threats'. Keep in mind that this core didn't even wall most attacks: a CB Heracross (especially when status'd and enjoying a guts boost) could still OHKO all three of these pokemon. Suicune could set-up against these three pokemon as well and pull off a 6-0 and although Zapdos couldn't hurt Blissey it could easily outstall it with resttalk. You were by no means safe, but you had a 'core', a way of dealing with most threats. This doesn't exist in bw2, which is why a lot of people have asked for 'the next Skarmory/Blissey'.
Moves; how they disincentivize switching in this gen
Not only pokemon are the cause of such an offensive meta, moves are as well. Switching isn't rewarded in the same way it was in gen 3 because moves are too strong. Let us take scald for an example: in gen 3, it would be a great tactical decision to switch in your Gyarados against a surf, allowing you to set-up or at least giving you a free attack. With scald, not so much: the 30% burn is just too strong. Swapping in your Tyranitar against Heatran? No thanks, lava plume has 30% burn. Moves have gotten stronger too, many times have I seen the words 'it's not very effective...' scroll uselessly across my screen as my pokemon just lost 60-70% of its health. U-turn and volt switch are also excellent examples of broken moves that give the attacker too much momentum. If I'm up against Rotom-W and am forced to swap out, he can simply volt switch, see what I swapped to and then swap in the appropriate counter. This was possible in gen 3 only by drypassing (baton pass first turn) and was the main reason for banning ingrain Smeargle, as Ninjask leads could bp the first turn and then bp to smeargle if they brought in their phazer or to something tanky like Umbreon to take the hit (who in turn usually sported mean look and taunt). U-turn and volt switch are apparently fair game now, however.
The next thing that disincentivizes switching is stealth rock: present in exactly 100.00% of competitive teams (maybe not on a very small amount of full baton pass teams), your pokemon are going to take damage upon switching in lest they have specific abilities and/or typing. Sleep has gotten a significant boost which made pokemon such as Breloom only stronger and once again disincentivized defensive rest-usage as switching reset the counter. Lets not forget that Draco Meteor and Outrage also pack a tremendous punch and are hard to wall - something we didn't have in earlier gens at all.
Let us not forget priority moves and new boosting moves such as shell smash and quiver dance, whereas you'd previously only got to set up +2 at a time (either by +1/+1 to stats such as dragon dance or calm mind or just +2 by swords dance), you can now set up faster by using quiver/shell smash. Just using swords dance and then abusing priority moves is also a viable option: you used to care about speed, even running jolly instead of adamant, but now you can swords dance with Lucario or Scizor and extremespeed/bullet punch away, not caring about the speed aspect.
Weather and how it benefits offense more than defense
It's quite simple: it adds another STAB to your water and fire moves. It does decrease the power of opposing water/fire moves but due to overwhelming moves (as explained above), +value is generally given to the offensive party rather than the defensive party, even when the defensive party makes something which intuitively should be considered a smart switch (i.e. an example of switching Gyarados into scald is listed above). Although rain may make Ferrothorn that much more dangerous by helping his x4 fire weakness, it also makes water attacks much more powerful against him. With rain up, certain things simply become close to or actually unwallable: choice specs Keldeo can hydro pump nearly every mon for respectable damage whereas a choice band Victini under sun doesn't care about what you swap in as long as it doesn't have flash fire: a CB Victini's V-create under sun does over 150 power off 328 attack against pokemon that x4 resist fire after weather, STAB and CB have been taken into account. To give you an idea: it OHKOs full health Gliscor.
New items benefit offense
Whereas we used to have only choice band, we now have things like choice specs, choice scarf, type gems, life orb, expert belt and whatnot whereas we only got extremely limited defensive items (eviolite is limited in its use, defensive berries are barely used, focus sash doesn't really help defensive pokemon but rather offensive pokemon). Flame orbs or toxic orbs to activate guts don't help the defensive side either.
Abilities
Abilities aren't that bad of an offender; things like Multiscale, Prankster, Thick Fat, Regenerator, Magic Guard, Intimidate, Rain Dish, Poison Heal, ..., ..., are all great defensive abilities that I feel are on par with their offensive counterparts like Moxie, Mold Breaker, Chlorophyll, Technician, ..., ..., The problem definitely isn't with abilities.
Conclusion
Today's metagame with a large distribution of powerful technician'd priority moves and lack of defensive options promote letting your pokemon stay in and die so you can revengekill the other pokemon. The power of switching is heavily diminished due to strong moves (either due to sheer power or strong primary/secondary effects) or simply strong offensive pokemon. The switch from gen 3 to gen 4 introduced the special/physical split by move rather than by type, making pokemon such as Skarmory or pure water types - who previously had no physical weaknesses - more susceptible to the type of attack they are supposed to wall, as the aforementioned Skarmory will die more easily to physical attacks now due to things such as fire punch, flare blitz and so forth, meaning that if you want to cover physical weaknesses you can't just run Skarmory and be largely safe; you need something that can wall physical fire/electric attacks as well. With defense nerfed and offense buffed (as well as the amount of threats being increased) it is very hard to play a more defensive, safe style as it is hard to properly prepare for everything that can be thrown at you, you simply don't have enough slots.
Suggestions
Weather: Drizzle, Sand stream, Snow warning and Drought should not be permanent. It's hard to give you a number, but I feel 10-12 turns would be appropriate, forcing you to bring your Politoed or Ninetales in more against weatherless, giving the opponent more chance of wearing it down. Additionally, 1.5x multiplier on fire and water is a bit too much, I'd prefer to see it lowered to something like 1.3x. Hail should also be boosted, perhaps get the 1.3x multiplier on ice. I'd also like to see some abilities that affect weather: Air Lock or Cloud Nine on usable pokemon or even entirely new abilities revolving weather. Additionally, I'd like to see new types of weather that benefit different types and provide unique mechanics, i.e. 'dark night' for dark pokemon, 'acid rain' for poison types and what not. This could provide any type of non-unimaginative-damage-boost(-as-seen-with-rain-and-sun)-bonus ranging from extra speed to an added chance to flinch.
Sleep: sleep buff should be undone.
Moves: things with a secondary effect such as scald (or even thunderbolt and flamethrower) should not be able to have their special effect triggered if the move is resisted. This allows me to actually swap in something such as Gyarados against scald and nerfs arguably 'lame' things such as Tentacruel spamming scald in rain.
Types: I'd like to see a new (defensive) type that is immune to dragon and has bonus damage against dragon and steel. This would allow defensive pokemon to not be set-up fodder for the enemy Ferrothorn or dragons and would make people think twice about spamming Outrage.
Team preview: I would actually like to see this removed. Usually, the more information the more skill-based a game is, chess being the ultimate example. With perfect information you do not have to make assumptions and you are never punished for 'good play', whereas in a game of imperfect information - such as poker - you may be punished for good play by losing to a hand which had only 8% chance of beating you. In a game such as poker where you play many, many hands per hour this is not as detrimental because eventually players who play well will be rewarded, but in a game of pokemon where most rounds are a best of 1 or at most a bo3, the good player may simply not always win. Providing more information in terms of team preview actually does not help: it simply turns into another rock-paper-scissors game where you can lead with pokemon A that would stop enemy pokemon X or with pokemon B that would stop enemy pokemon Y, with pokemon A losing to pokemon Y and B losing to X. A lot of good leads are also rendered useless due to team preview and actually enables people to spam their outrages: they know your options. Explosion's damage was essentially halved, but in gen 3 it wasn't a big problem due to no team preview: a CB Metagross lead in gen 3 could OHKO nearly everything (even some Skarmory) but wasn't a dominant strategy because there was no team preview, you had no idea what the opponent was packing.
Moves: U-turn and volt switch should force you to select your switch-in before using them so you cannot see what your opponent is going to switch in and then appropriately counter that switch-in without losing any value. They should also do less damage to a substitute: if you are stuck in an unfavorable match-up (i.e. Rotom-W vs. Breloom) you should not be able to switch out and break the substitute at the same time, it takes away from the power of substitute and prediction. Right now, u-turn and volt switch are just 'hurrr, always +value'-moves.
Pokemon teams: I'd actually like to see 7 or 8 pokemon being the maximum in a team rather than the usual 6. This may rustle some 'nostalgiafags' but I think it would contribute to a healthy, innovative metagame.
Will-O-Wisp and Toxic: I'd like to see Toxic finally getting 100% accuracy and would love to see Will-O-Wisp boosted to at least 80%, preferably 85%.
Stealth rock revamp: GF needs to make up their minds about what they want SR to accomplish. Is it simply a hazard to punish flying types who are immune to spikes? If so, have it only affect pokemon who are flying or levitating for 25% of their health (the same as 3 layers of spikes). This would make SR optional instead of a must have on every. single. team. Do you want stealth rock to be like spikes? Then have the damage capped at 1/8th maximum damage while keeping the 1/16th and 1/32th damage done to resistant pokemon so there are at least some counterplay possibilities.
Rapid spin and spinblocking: I'd like to see another move - maybe even 2 moves - that accomplishes the same thing as rapid spin. This attack should be of another type, i.e. a ghost type or should simply be useful rather than being useless offensively.