Well, lets see: They've gotten rid of the divekicks for most if not all characters, less combos/attacks ,only x's, whereas in the first game you were able to do different combos if you pressed y during x's, whole gameplay feels a bit more focused on getting your form from the powerstones because base forms feel weaker in this game (for me) plus more powerstones spawn (can be changed via setting though, i know).
There is an actual time limit when it comes to beating the final boss, if you don't, then you get OHKO'd. If you die at any stage during adventure mode, then thats it, you can't continue from there. Arcade/Adventure mode feels very short unless you play the 4 player version of arcade. I still enjoy playing the game but I would be lying if I'd say that it doesn't have any issues.
I thought I wasn't the only one who didn't like Power Stone 2. Yeah, it felt like the developers at the time was going near Smash Bros. territory, losing the characters' uniqueness with the variety of attacks/combos from 1. Hell, even the transformation supers were a bit lacking.