Hmm...The ORAS metagame...where should I start? I do say that starting with X/Y, the nerf of weather was a huge improvement. The meta in BW or BW2 was very much unbalanced to the point weather was used throughout the entire battle. Drizzle, Drought, and Sandstream were often spammed giving Fire, Water, and Rock/Ground/Steel types a lot more play. Sandstorm was very annoying getting off residual damage every turn when you don't have Leftovers. Generation 5, rain just dominated the metagame. Genesect was absolutely horrible. Ferrothorn had grown more useage with the help of Politoed, along with Scizor. Additionally, Ferrothorn was more annoying last Gen than 6th Gen. With access to Dream World, made Politoed and Ninetails the most annoying Pokemon to deal with. I do remember there was some discussion about banning teams that have both Drizzle + Swift Swim together. They've become highly more viable in OU. Excadrill made a huge impact to Generation 5 because of Sand Rush having the exact same effect as Swift Swim did and with the help of Tyranitar or Hippowdon, it was almost a nightmare for other Pokemon to deal with. After a Swords Dance, it's over and it was to the point it had to be an uber Pokemon Gen 5. Hmm...the Rotom formes getting types in Gen 5 was kind of nice as well. That way, they don't have to spinblock when there's hazards on your field because at the start of Gen 4, all formes were Electric / Ghost types. Psyshock was helpful in putting a stop to bulky special walls like Chansey and Blissey.
Just all together, Dream World Abilities made a huge impact and changed the metagame a lot. Making Pokemon that were completely terrible such as Sableye for example, good now. Generation 5 introduced a long-term planning and strategy with good teammates. There has been a huge variety of playstyles such as hyper offense, balanced, stall, bulky offense, and much more. Weather had made many Pokemon viable due to it extending the entire battle.
Another thing I would like to add is that Dragon was seriously annoying, especially when I was in the Battle Subway in the Wifi train battling against players that had a spam of Garchomp, Salamence, Dragonite, Haxorus, etc. It was just a pain to deal with Choice Scarf Dragons and it wasn't as fun as I would expect it to be at all.
With X/Y introduced Fairy-types, thank god. Those beautiful little Pokemons tamed those annoying Dragon-types. Talonflame was a huge mess to deal with. That thing was able to deal with many types along with Gale Wings, excellent dual typing, and given it has Swords Dance was horrible. Mega Kanga and other OP mons went bye bye. I do say with the added Mega Evolutions, made some types more viable than others. Defog became another entry hazard remover, which Flying-types became more usable. Generally, Steel got a huge buff because of (I don't know when Aegislash was type banned nor did I have experience with it on Steel so idk) Mega Mawile, Mega Aggron, and Mega Scizor. To me, I know that I recently got into Monotype as of November 2014, but that type was incredible. Playing on the ladder with Steel, I ran into a bunch of opposing Steel-types, Fairy, Fighting, Flying, and Water (Mid 1500 mark).
ORAS, I feel is much more of a breather to various types in the metagame. Besides the Pokemon that were banned such as Mega Metagross, (excluding rebanning Mega Mawile), Mega Slowbro, Mega Altaria, and Mega Salamence, and Greninga it has given more teams to function better and the metagame more balanced. Smooth Rock was the most important thing I really wanted to get out and have Monotype look at. I saw how terrible it was when I laddered due to the Hippowdon+Landorus-I+Excadrill core. With the new Mega Evolutions, Normal-type finally got itself out in the open tbh. I forgot that with Mega Kanga not being allowed, Normal was actually the only type to function without a mega until Mega Lopunny, Pidgeot, and Audina came along.
With the banlist that we have and the sorts of moves that were introduced in ORAS such as Gyarados getting Crunch for example in a way made things a little more fair for all types. We're able to freely use and explore more sets that could be proven viable. I remember testing out Calm Mind + Rest Sylveon not too long ago and it was okay to say the least.
http://replay.pokemonshowdown.com/monotype-334997479
http://replay.pokemonshowdown.com/monotype-335240045
http://replay.pokemonshowdown.com/monotype-335218225
My favorite part was ORAS bringing more diversity to the table. It allowed for us to explore more sets that could work in the current metagame along with actually planning out strategies. I guess I already explained why it was better than the previous Gen already.
One change...I don't think there's really anything I do want to change about the metagame. A while ago, I was really looking forward to a Hoop-Unbound suspect which it didn't get. I hope that Sun/Moon really brings something that'll help other types (one example is actually make Ice resistant to water...or make them neutral to Rock).