The characters don't have nearly as much personality as the original. For example, Sly was perfect being the anti-Ty yet also his brother, but in the newer games he's become just another Ty, being all friendly and crap. Ty himself just repeats the same tired phrases over and over again.
The gameplay is also a hot mess. While 60 fps is an improvement, the way it's all structured is just overly complicating considering the original's format.
Forgettable soundtracks on both ends as well, while the original's soundtrack is very memorable.
Really, I don't understand why the sequels get any love. :/