In my opinion...
Zelda - Din's Fire of course and Faore's Wind being awesome too, but her smashes and tilts all feel much more accurate now. They all feel like they "suck" the opponent into them as opposed to before where they felt like they pushed away from the sweet spot of the hits. Her upair is fast now, and her downair is a b/fair pointing downwards now as it always should have been. Naryu's Love now actually does its job of pushing someone away very well, in Melee that move was so finicky. Before Zelda fans in Melee get all crazy it's not to say she wasn't potentially good in Melee too, but I feel like in Brawl she's better in almost every way. Her Fair and Bair are a little harder to sweetspot now but not by much, it's a small price to pay for all the plusses she got. Everything she does now fits her intended style perfectly. If only the transformation were just a -tiny- little bit shorter...
Game and Watch - I don't play the character much so I can't be too in depth here but even in my limited time with him he does feel much more potent. I think it's mostly his aerials. It's like they got a mad amount of priority all of a sudden. He generally just functions better though...maybe it's more the new engine more than anything character specific but whatever it is it worked.
Kirby - Definitely improved. New Yo-yo Kirby inspired dash attack is way better than that fire kirby move, Hammer move is a little more bonafide for regular use, and in general everything he does doesn't feel as weak. The hits that are meant to be low knockback do their job, and the hits meant to knockback further than others do it just right.
Donkey Kong - I don't play the big DK much but I can see why he's regarded as such a threat when he doesn't have some frightening Tech that's abusable against him. His throws latch on to you very well and his hits just feel more accurate. He's one of the few characters that actually benefitted highly from the low hitstun. I'd say even if future games had hitstun put back in heavier characters like DK should still be less suseptable to it, it's the key I think to making the heavyweight characters contenders instead of airborn punching bag hard-mode characters.
Bowser - I know a lot of people are gonna get on my case about this, but in my personal opinion I think he got a little better. While I miss the biting too, I think the new piledriver move he does for forward B is way more effective than that shoulder tackle he did before. If only he bit then did piledriver in this...that'd be the best of both worlds...
But yeah, also his fire breath pushes away a little better now too and his forward smash comes out a little quicker. Even Bowser Bomb if you're actually hit by it feels like it hurts a lot more. All and all I feel like he's generally a slightly better character. If you prefer the way he was in Melee that's fine, I'm not looking for an argument. Just based off of my experience between the two versions I feel like Brawl ver. has a few basic advantages. I do miss using L canceling with his stuff too a bit but I'm talking about the bare-bones characteristics.