I wouldn't say I don't like any specific thing out of the game, but there are a couple things that brought me disappointment..
1. Diddy wasn't made with the greatness I had in mind for him.
2. Most of the stages don't seem to be very big. Dynamic, yes, but just not big enough. I mean, look at the size of Castle Siege (the first form). Can't even fit two pikachus on that thing.
3. The stickers will just cause me annoyance, as I'll get all OCD on myself if I don't manage to find them all.
4. The Subspace army seems a little random. I would have rather seen something a bit more grand.
As much as this game isn't perfect, it has already far surpassed Melee in (I'm sure) everyone's eyes, so we really shouldn't be looking at the flaws like this..