Basically ALL of what Dahbomb said up there ^^^^^^^^^^^^^^^
EDIT:
Also some character MAY of gotten nerf to hell which was NO reason for, but others became GOOD because they needed buffs to become playable characters
Chris: Couldn't cancel his c.H or j.dH at all, making this OTG useless unless he had assists to help him continue the combo, and his mobilty was ASS, but they made him better because of the overall changes that he NEEDED to become better. being able to cancel his gunshot normals, being able to cancel his dash, better scaling on his special moves, and made his j.S better too, because of this he is in the top 20 in this game.
Haggar: In Vanilla he was an Assist only character, and if he was the only one left, you most likely lost the match no matter what, because he had no OTG, he needed an OTG assist to continue the combo for him, and his damage output was TERRIBLE, and having no invincible hyper at all. In UMVC3 he got a OTG move, his c.H, that he able to continue the combo with assists, able to now combo into his OTG hyper a lot easier, because of his OTG it made his damage output amazing, also they gave him a better airthrow that he can combo from it easier now, and gave him an invincible hyper too.
Arthur: Even thought he is still low tier, he is TONS better then what he was in vanilla, most consider him the worst character in vanilla and I agree, because he had no mobility at all, TERRIBLE jumping normals, and other shit, in UMVC3 they made his j.S amazing, and also gave him mobility at the same time by being able to cancel it into specials and making him fall faster during it, and making it able to OTG, and they also gave him more health also, in this game he is able to now fight against rushdown because of his better mobility and also giving him a DP move that is invincible too!
Others that they fixed:
Akuma: made him so he can be used an a point character and gave him better damage output
Trish: gave her an OTG hyper to fix her damage output.
Captain Amercia: Gave him easier damage options, and an a OTG
Deadpool: Increase his damage by giving the ability to cancel his teleports from his specials, and gave him a better airthrow.
Dormammu: Gave him an OTG to help with his damage output, and made his spells usable by coming out after they get charged fully, and other various buffs too.
Thor: Gave him armor on mighty strike and decreased the charging time for it, gave him better damage scaling, made his command grab hyper useful, and made his command grab specials better
There are characters that got overbuffed, but overall there are characters that were terrible/incomplete characters in vanilla that got TONS better because of buffs/fixed that they needed to become usable in the game.