When work first began on the Pathfinder RPG, I set down a number of principles to guide me. Since this game is based off the 3.5 rules set, I wanted to make sure that it stayed true to the original vision of the game. When taking a look at these rules, please keep the following guidelines in mind as they might help you understand the changes that were made.
Improve the Game: The 3.5 rules set is excellent, but it has its flaws. Over the past few years, a number of common problems have seemed to crop up again and again, problems that delay the game or cause no end of arguments (grapple and polymorph, for example). I wanted the Pathfinder RPG to clean up these rules, by streamlining in places and adding options in others. You can still grapple in the Pathfinder RPG, but it is no longer the huge headache that it was. I also worked to even out some of the choices. A number of 3.5 skills are far less valuable than others, making them suboptimal choices. In my experience, few rogues took Forgery, but Spot was an incredibly common choice. These rules work to even out some of these choices. So while you might still take Perception over Linguistics, the latter is now a far more useful choice than it was before.
Add Options: Just before design began, a friend of mine asked me why no one ever seemed to take rogue beyond 2nd level or fighter beyond 4th level. This got me thinking. Far too many of the basic classes lose their luster after just a few levels, leading most players to take a host of other classes or a number of prestige classes. While this option is still available, I wanted to add more to all the classes, to give each one of them a reason to be followed up through 20th level. To this end, I have tried to add options to the game whenever possible. This is a tricky path to walk, though. Too many options and you end up with overly complicated classes. Too few and players get bored of their class. This principle goes beyond class powers, as well. From sundering to magic items, there are now more options and choices to make than ever before, each one opening up whole new avenues of character and adventure design.
Compatibility: Of all the goals I set out with when designing this game, compatibility ranked near the top. I wanted to make sure that any rules we changed were adaptable to the extensive body of work that exists for the 3.5 rules set. In addition to being compatible, I wanted to ensure that any conversion work would be minimal. In most cases, this meant adding to existing rules, instead of subtracting. So, while we changed the way turning undead works, we did not remove turning undead from the game. We added options to the fighter without removing any of them. This design philosophy doesn't always hold true, however. Some skills were combined and a few disappeared altogether (goodbye Use Rope). Whenever I broke this rule, it was because the other guidelines took precedence.
(With that said: Were epic levels for anyone at all? 
