I think most people acknowledge that Generation III OU was a good metagame. I think most of the changes that undid its balance were actually before BW back in DP, namely:
1. Physical/Special split: This made Pokemon make more sense, but it powered up offensive Pokemon lot, and things would be WAY different without this. Gengar wouldn't have a STAB. What happens to Garchomp and Salamence if they only have one physical STAB? There are a ton of examples. The p/s split greatly increased the versatility of offensive Pokemon, giving them more options and making sure that none of them were STABless. This was huge.
2. Power creep: Manifests itself in three ways...
A. Pokemon: Howdy, Garchomp. Not unique to DP though--happens every generation, seemingly.
B. Moves: needless to say things would be different if Outrage was still 90 BP and Stone Edge and Close Combat were still Rock Slide and Brick Break. And Draco Meteor is just unstoppable. Nasty Plot makes me weep. Sometimes you wonder if the Pokemon are really that much stronger or if they just have better tools to work with than before. But this was very much a DP thing.
C. Items: Choice Specs, Choice Scarf, Life Orb, and Expert Belt did not exist prior to DP. Suddenly, you had a perfect lure item (Belt), the perfect set-up and mixed sweeper item (Orb), and a supplement to Choice Band.
3. Stealth Rock: This works both ways, but still, losing so much health (cumulatively) is going to make battles go quicker. This doesn't mean Stealth Rock is broken, it just means it fundamentally changed the game, and I think to the benefit of offense. Defensive Pokemon often can't afford to lose that extra 12.5%, while some sweepers can clean with 45% health, like a SDLuke, for instance.
4. More threats: I feel like this one is kind of easy to forget (though this is something that obviously happens about equally with each generation), but it's worth noting that the more threats you have, the more impossible stall, a fundamentally reactive strategy, is to run. I wonder if just by coincidence, DP saw the beginning of where many Pokemon would be so viable that you couldn't possibly cover them all and had to just kill before being killed. This development was delayed in the banning of three (!) Dragons at the end of the generation (Garchomp, Salamence, Latias). Still, when this happened, I think Pokemon shifted toward being about team building and team matchups. In-battle execution is still important and still swings many battles (this is why competitive Pokemon is still appealing), but it's certainly easier to slug through without actually needing to play well just by virtue of the powerful weapons at any given player's disposal. It can be a bit of a crapshoot is all. I don't see that changing, to be honest.
I think Gen 5 just reintroduced or introduced Pokemon that could capitalize on all of this, seeing the return of Garchomp, Latias, and Salamence, the additions of Haxorus, Hydreigon, and Latios--plus Terrakion, Reuniclus, Excadrill (banned obv), T-Dawg (ditto), etc.--and auto-weather obviously gave power-ups to many Pokemon and was the major difference between gens four and five.
I don't think competitive has necessarily gotten worse; it merely is not the same game as it was two generations ago. Different skills are required to succeed in BW's game than RSE's. I would say that both are fundamentally competitive, though.
If we want to make BW OU more like Advance or late DPP, I think you're on a fool's errand, to be honest. It is what it is. I think some steps that could limit new elements of the game would be banning auto weather and banning Stealth Rock, not because either are "broken" (I am exasperated trying to define what that means) but because they help offense so much. From there, I think you'd have to evaluate where the game is, but I suspect it would merely look like DP and would still suffer from issues of Dragon centralization and Terrakion-like powerhouses. You'd probably have to ban a lot of things that aren't *really* broken but would even or balance out the game. But then your governing of Pokemon is entirely subjective and is as arbitrary as the game itself can seem, which is why I think you just have to accept it as it is and either play it or play a lower tier/Ubers or a different gen.
(If you're not afraid of changing game mechanics, removing critical hits, lessening the power of certain moves, etc. are considerations. Good luck getting consensus on that though.)
