Gained:
Specs Lucario
Many people may be saying "what the fuck, SD Life Orb is where it's at!", but really, look at Specs Lucario again. All the SD Life Orb rage in the last 4 months has really opened the metagame up for Specs Lucario domination. All of the things sent in to beat SD Lucario are beaten by specs. Actually, everything sent in to beat Specs Lucario is beaten by Specs, so yeah.....
Garchomp
Garchomp went from really hyped, to mediocre, to the most dominant attacking force in all of OU. So dominant in fact, there have been arguements to make it uber. Sand Veil is a trait that is arguably one of the best, forcing 100% accurate attacks to drop down to Stone Edge accuracy in the sand. Not to mention Garchomp has the ability to 2HKO literally everything in OU. Garchomp does it all, with just a few attacks.
Shuckle
That's right, Shuckle. With Sandstorm boosting a 0 SpDef Shuckle's special defense to over 700, and having 614 defense with a decent (not great, but OK) defensive typing, Shuckle has really become a force in OU. Encore is one of the most deadly attacks a team can face, considering it gives your opponent a free turn to do whatever. Add in Knock Off and Toxic, and Shuckle can actually take out several pokemon on his own. Only downfalls are that Shuckle is completely owned by Taunt, 244 HP and Rest as a recovery move.
Heatran
Remember the discussions of Heatran being BL? I do. Heatran is another dominant force in OU. People have realized that a 4x weakness to ground is not all THAT bad, especially with all the great resistances Heatran gets with Steel + Fire. Not to mention great defenses and HP, as well as a monster SpA stat. Heatran is really the complete package. It can wall on a defensive team, be scarfed or speced, beat Blissey via Taunt, and has a great ability. What's not to love about Heatran?
Gengar
Gengar was good at the beginning of DP, it's just insane now. So many different sets and capabilitys, Gengar is threatening just entering the match simply because your opponent has no idea what attacks it's running besides Shadow Ball. Not to mention great SpA and speed, Hypnosis, and some great resistances. Gengar has went from good to a rival of Garchomp in terms of usage, utility and deadlyness. Scarf Gengar is everywhere, and this just makes other sets more threatening.
Deoxys-E
A (almost complete) move down to OU, and Deoxys-E has become a underrated star of DP. It's really THE pokemon to bring out late game. Being able to nearly max both attack stats while outspeeding almost the entire metagame is something very valuable and unique to Deoxys-E. A movepool deeper than aldaron's vagina, Deoxys-E is a big threat. Base 90 attacks are considered "mediocre" by many, but when Deoxys-E is hitting everything SE with 95+ BP attacks, it's not so weak anymore.
Celebi
Wow, who ever thought Celebi would be used so much? Base 100 defenses are quite bulky, and Celebi gets Recover as well, something other bulky pokemon dream about. Celebi also has rather good SpA, and STAB on Grass Knot. Thunderwave, Reflect, Stealth Rock HP Ice, Substitute, Baton Pass, Leech Seed and U-turn are all great attacks to choose from when finishing up the moveset. As many people have found, Celebi makes a decent counter to Gyarados (though still loses to CB Ice Fang and DD Ice Fang when Gyarados is at 100%). Celebi has found a way onto many teams because of great utility value, while still being able to do good damage to a lot of pokemon.
Zapdos
Much like Celebi, Zapdos is surprise to many this gen. Finding a niche as a pokemon with great offensive and defensive balance, as well as a great counter to pokemon such as Gyarados, Lucario and some Garchomp. Zapdos is one tough cookie. Roost is a great asset to the bird, and a STAB'd Thunderbolt does a great amount of damage.
Loss:
Azelf
Sadly, one of my favorite DP pokemon started out huge, and got worse as the metagame evolved. Azelf started out as a monster, 125 base SpA, 115 base Spe and Nasty Plot. Throw in some great attacks, and you've got yourself a good pokemon! Well, that was right at first. Sadly, as the metagame evolved, the speed standard kept getting higher and higher, until Azelf couldn't keep up. And unlike Gengar, Azelf really isn't to versatile to make up for the growing ineffectivness at sweeping.
Porygon-Z
Another one of my DP favorites, Porygon-Z started out strong like Azelf. Nasty Plot, 135 base SpA, respectable speed, and the ability to 2HKO Blissey with Tri Attack. Porygon-Z was predicted to be another monster of DP. Unfortunatly, Porygon-Z went downhill even faster than Azelf, but for the same reason. As the speed game went up, Porygon-Z's effectivness plummeted. A scarf was the best choice, but even then, Porygon-Z is completly stopped by standard special walls. Just like Azelf, Porygon-Z just doesn't have the utility to keep being usful.
Blissey
Still a strong force in OU, but most of the used special attackers have found a way around Blissey. Not to mention, the lack of dealing damage isn't that great. Blissey still has merits with Thunderwave, Sing and Toxic, but other pokemon can do that as well, and do a quite a bit more.
Cresselia
Starting off as something that was thought near impossible to take down, Cresselia has taken a huge hit through the evolution of DP. In the beginning, Cresselia could use Moonlight with 3 attacks and monster defenses. Now, two moveslots are almost needed for Rest and Sleep Talk. Giving Cresselia a status really hinders her ability to do anything except sit there in battle and take hits. And if you don't run a status, Cresselia's special attack isn't high enough to OHKO Garchomp or Gyarados, two pokemon she started out as a great counter to, and these pokemon are able to take full advantage to stat up and beat her down.
Donphan
Started out really strong as a good defensive pokemon, and Rapid Spinner, and has died off to almost non-existant in OU. Donphan's lack of recovery, horrible special defense and lack of power compared to other walls/rapid spinners has sent him spiraling down further and further.
Breloom
Breloom was once something even the best players feared, with great attack, Focus Punch, Poison Heal, Leech Seed and most of all, Spore. However, with the advent of Celebi usage, and the speed increase, Breloom is more or less dead weight on a team now. And in a metagame dominated by late game sweeps, Breloom has dropped even further down with those horrible defenses and speed.
Disagreements:
Rhyperior
Rhyperior is really not as bad as everyone thinks. Great HP, attack and defense along with Solid Rock are still extremely formidable. I've read some posts about how Rhyperior can't get past Hippowdon or other physical walls. That is a straight up lie. Running an Adamant nature, and 196 HP / 84 Atk / 168 Def / 60 Spe, Swords Dance and Earthquake, Rhyperior outspeeds and 2HKO's Hippowdon after 1 Swords Dance, while Hippowdon does a paltry 30%-35% with Earthquake. Rhyperior's typing is disapointing, but it can take a surprising amount of punishment before going down. Just leaving it in against special attacks is obviously dumb. However, Sandstream boosts Rhyperior's poor SpDef to a respectable level, high enough to actually be 3-4HKO'd by Ice Beam.
Electivire
People seem to dismiss Electivire because it's not as good as they once thought. Well, that's right and wrong. Physical Electivire really isn't as good as we all thought, and has plummeted in usage. However, take a look around the RMT forum. Flamethrower / HP Ice / Thunderbolt / Cross Chop absolutly rapes most teams, espcially with a Motor Drive boost. Why people don't use this more is beyond me.
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There you have it. I could have kept listing pokemon in the improved section, but I do have other stuff I need to get done tonight.
EDIT- I wrote all of this to try to bring some discussion to this thread, which has been complete ass so far. However, it has potential to turn into a decent thread, so please, let's actually talk about something and not let the last half hour of my life go to waste!