In short, I would answer your question like this:
- The focus of PRPG is to retain compatiblity with 3.5 edition as far as possible, but to remove/replace elements that proved troublesome in play. More 'ease of use', better scaling of class abilities, and an attempt to bridge the power gap in between classes.
- As far as I know, no comprehensive list of changes exists (sadly), but beause a lot of minor tweaks have been made, I would suggest you download the (free) Pathfinder RPG beta and see for yourself. Key points include a revamped skill system, faster feat progression and general reworking of all classes' 'key' abilities.
Also, since during the Playtest there has been some changes on the PDF version, I suggest you to take a look at the excellent summary of changes made by Charles Evans 25 here.
