Alakazam-Two words: Magic Guard. Now Alakazam can run around throwing out Life Orb-boosted attacks with no drawbacks, not to mention the immunity to all other forms of passive damage. That means that the Sub + 3 Attacks set is all the more deadly.
Cloyster-Shell Smash makes Cloyster easily one of the most threatening Pokemon in OU after just one turn. Not just that, but the power boost to Icicle Spear gives it a very powerful STAB move. It can even soak up a few physical attacks if it needs to, provided you're running White Herb.
Deoxys-D-Wait, it dropped from Ubers into BL, how did it improve? Well, honestly, who was using Deoxys-D in 4th Gen Ubers? The fact that it was allowed into standard for its trial run is a much needed improvement for it. Even though it hasn't quite made the OU usage cut yet, it is still a very valuable Pokemon for many OU teams, especially serving as a replacement for the previous favorite of HO teams, Deoxys-S.
Donphan-Well, it got an improved Sturdy, but the more important thing is that Drought happened. Donphan is one of the best spinners for Sun teams because of its unique assets (STAB Earthquake, Ice Shard, etc.) and the fact that the Sun doesn't actually hinder it like other spinners.
Dugtrio-Instantly trap an opposing weather starter and kill it off? Yes please. Dugtrio is one of the greatest new tools to win the weather wars with Arena Trap and its STAB Earthquake.
Gastrodon-The fact that Gastrodon has both an Electric and Water immunity, actually benefitting from Water attacks, makes it one of the best checks to Rain teams that there is. It also makes it a near-perfect wall to Pokemon like Rotom-W and Vaporeon.
Gengar-With the upgrade to Disable, Gengar got a whole new niche with SubDisable. Not a huge improvement, but there's also the fact that more Fighting types in OU makes the Ghost typing a bit more valuable.
Gliscor-Again, Poison Heal is a real blessing. Kinda went through that one earlier.
Heatran-How did Heatran improve? Well, think about it. It didn't really get its chance to shine last Gen until Salamence was banned and was no longer a threat, but even now it is one of the most used Pokemon there is. First of all, Drought in OU makes Heatran both an excellent abuser and check to Sun teams. In addition, the Air Balloon makes Heatran a much better answer to Pokemon like Gliscor, Landorus, and Salamence.
Infernape-Iron Fist makes Mach Punch a much better asset, and just wait until it becomes legal with Thunderpunch and Fire Punch. Not only that, but because of the way it functions, CB Infernape is now one of the most effective sets it has, when it was more of an afterthought last Gen.
Jolteon-Volt Switch is one of the best new toys that Jolteon can take advantage of. Not only that, but its speed is even more important in this metagame, allowing it to revenge kill fast threats like Landorus, Tornadus, Salamence, Terrakion, etc.
Lati@s-Like Deoxys-D, these two mainly improved because they were given another shot at OU. With the Soul Dew, they were still used in Ubers last Gen, so it's not like they weren't getting any love. Still, it's a whole lot better for their chances in standard OU when...you know, they're there.
Mew-Again, he has his chance in OU like some of the others, and he's proven to be a nice wallbreaker, Nasty Plot sweeper, whatever you need him to do really. He's Mew.
Nidoking-Oh yeah. Sheer Force. I'm really hoping that the no-Life-Orb-recoil-with-Sheer-Force thing isn't a glitch, because it'd suck if they "fixed" it in BW2. Anyways, running around with the fantastic coverage of Earth Power/Flamethrower/Thunderbolt/Ice Beam, a Life Orb boost, and no LO recoil is an amazing thing.
Quagsire-I don't know how this thing is still NU. It's easily one of the best answers to random setup sweepers in the game, and now it gets to throw around random burns with Scald. Come back from last Gen, where it was pretty much a niche counter to Kyorge and nothing else, it's quite an improvement.
Rotom-A-We all know that Rotom-W, with its great typing and Levitate, is one of the best OU contenders right now. In addition, the fact that the typing changes caused the Rotom-A forms to be different enough to split up between tiers means that each tier now gets to have their own Rotom-A fun, so they all sort of improved. But mostly Rotom-W. Because it's awesome. Hoorah.
Sableye-Prankster baby. Coming from the Pokemon who got Stall, which is...just awful, Prankster is a Godsend to Sableye. Priority Will-O-Wisp, Taunt, Substitute, and Recover is awesome, making Sableye one of the best stallbreakers and checks to random sweepers out there.
Sharpedo-Speed Boost with a base 120 Atk stat is great, especially with Rain boosts to Waterfall and Aqua Jet. STAB Crunch and coverage in Earthquake also help. Heck, you can even go mixed with a base 95 SpA and moves like Ice Beam and Hydro Pump. See, Sharpedo just couldn't take advantage of all that last Gen, but now he can.
Slowbro-Leftovers, Slack Off, Regenerator, this thing just doesn't want to die at all. It's easily one of the most effective physical tanks in the game, boosted by the burn chance from Scald. Coverage moves like Flamethrower to take care of random Grass types (looking at you, Ferrothorn) gives it an edge over other bulky Waters.
Tangrowth-Is there an echo in here? Leftovers, Leech Seed, Giga Drain, Regenerator...again, doesn't want to die. Tangrowth is even more physically bulky than Slowbro, with the ability to avoid a 2HKO by any of CB Terrakion's attacks. Its Grass typing also gives it a unique set of resistances that other Pokemon just can't boast, and it's deceptively strong on both ends of the spectrum as well.
Toxicroak-Okay, so Toxicroak didn't change much at all. What changed is that Drizzle dropped into OU. Now, between Leftovers and Dry Skin, Toxicroak is restoring nearly of a quarter of its health every turn in the rain. That also means that Toxicroak can run Life Orb and actually restore more health than it loses to the recoil, making it a great Rain sweeper.
Venusaur-Seriously, Chlorophyll is just a fantastic addition to Venusaur's arsenal. Now that its speed gets doubled in the Sun, it can wreak havoc with Growth-boosted attacks and a super-fast Sleep Powder. HP Fire also gets a pseudo-STAB in the Sun, making opposing Grass-types run to their mommy. And you could honestly throw all Chlorophyll sweepers from 4th Gen and earlier in there too, since having Drought in OU made most of them viable in the standard metagame.
