Japanese gaming magazine Dengeki, through PSPHyper, has some information about the nature of Dissidia and why the decision to bring it to PSP rather than the PS2 was made. Yousuke Shiokawa and his team seem to be a focal point behind this game and it was decided that their expertise was "the best way to gather all the characters from the Final Fantasy series".
Shiokawa was lead programmer of boss battles in Kingdom Hearts II and will be directing Dissidia. The report also goes on to state that the PSP's ad hoc mode was the reason it was chosen as the platform.
Tetsuya Nomura states that if they were trying to aim for a large user base then the PS2 would have been the preferred option, however if they had it would have meant splitting the screen for multiplayer which in their eyes would have diminshed the fun.
It seems that the battle system will borrow heavily from the Kingdom Hearts franchise, whether thats good news or not thats up to you to decide. However they will be building upon this but expect the regular mainstays of levels, class, weapon upgrades etc.
Source:
QJ.NET