Here's a pretty thorough
rundown of most of the changes between PF and 3e.
The biggest changes to keep in mind:
-Combat Maneuvers: The mechanics for special attacks like Bull Rushes and Grapples have all been rolled into a single unifying mechanic. Every character gets a Combat Maneuver Bonus and Combat Maneuver Defense, and these special attacks are essentially a check of one against the other.
-Skill Points: Instead of getting 4x the number of normal skill points at first level, characters get the same number at each level. The first rank you put in a class skill gets you a +3 bonus to that skill. You can normally have a maximum of one rank per character level in a skill.
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[edit] Feats: I almost forgot about feats, which are now earned at every odd-numbered level.
-Concentration: Concentration is no longer a skill; instead, it's a caster level check.
-Archetypes: Introduced in the Advanced Players Guide. Think of these as a suite of thematically related alternate class features. As GM, you should perhaps consider what archetypes you would permit or prohibit.
Here are some general things to look out for:
Race and Class Power Boost: Each core race and class from 3e has a number of new powers in Pathfinder. Most races gain an additional +2 bonus in some skill. Most classes get a "capstone" ability at level 20, to encourage focus in a single class. Pathfinder designers also strived to avoid "dead levels," instead letting classes gain a new spell or class ability every level.
False Friends: Pathfinder is relatively easy to convert from 3e, but the number of near-similarities between certain common features might actually make it harder for your players to learn if they are very familiar with 3e. Be sure to check familiar feats, spells, and class powers to be sure there isn't some sneaky difference between 3e and PF.
