The game 'looks' so much nicer for a start. The artwork is exciting, vivid and fresh. They have apparently (so I've been told by a number of people) cleaned up several bits from 3.5 and made it a better game (although 3.5 was pretty much fien as it was).
Plus being OGL in itself the Pathfinder game will attract a lot of the third party publishers that aren't wanting to follow, or circumnavigate, WotC with the 4E GSL.
You can download the Alpha Pathfinder RPG Playtest rules free of charge from Paizo's website here (you will need to create an account though).