So, it's no secret that Pathfinder is everybody's favorite new toy, but with so much new/modified content, where do you begin? (Be sure to check out the PRD even if you don't own/haven't ordered the books yourself!)

10. Streamlined NPC Statting: Similar to the tables in the back of the v.3.5 Player's Handbook II, but taken a step further, the NPC creation rules are an invaluable resource offered by Pathfinder, and could be easily adapted for use with v.3.5.
9. No More XP Fees for Spellcasters: Spells and magic item creation no longer costs XP, removing what was one of the larger roadblocks to these options in the past. Spells now have costly material components to make up for the lack of XP cost, whereas XP has been removed entirely from the equation for magic items (which should allow you spellcasters out there to save enough money on magic items to buy all those expensive components I mentioned).
8. Better Monks: Monks have a better flurry of blows and can perform combat maneuvers with the best of 'em now (even better since the huge size advantage monsters of Large size or larger had over PCs has been all but eliminated), in addition to a bunch of cool Ki pool tricks.
7. Better Fighters: Fighters have more than one class feature! Yay! They don't exactly have the best variety of class features around, but there are a lot of new, tasty feats to sink your teeth (and by "teeth," I mean "numerous bonus feats") into.
6. Bloodlines/Domains/Schools: I love all the new powers granted to spellcasters through their choice of these options. All of them grant pretty unique and defining abilities, and I'm exited to see them in use in-game.
5. Non-Useless Favored Classes: I'm pretty sure the gaming community at large ignored the v.3.X favored class system, so it's refreshing to see a system in place that I won't have to pretend doesn't exist. This also encourages more fidelity to one class (though not prohibitively so), which I really like.
4. Skill-Based Magic Item Identification: I've liked the idea of using Spellcraft to identify all manner of magic items ever since WotC suggested it so many moons ago. I've been using my own houserules to implement this in my games, but I was glad to see Paizo adopt this method of identification as well.
3. Simplified Skills: I like the 4th/Saga skill system, but it does have a way of generic-fying all characters in that everyone is pretty good at everything. While I don't want my player's characters (or, gods forbid, my own) to fail horribly because I thought Perception would be more useful than Acrobatics, it does create an air of dramatic tension to have flawed characters, characters who aren't good at everything. That said, I believe that Pathfinder's simplified system ("simplified" in that they've combined the easily combine-able skills and gotten rid of the overly-confusing cross-class skill system) looks like a noted improvement over v.3.5.
2. Non-Broken Polymorph Spells: Luckily, I've never had to deal with polymorph as a DM, but I've many horror stories. I'm glad this one's been revamped (in a way that seems to be the spiritual successor of the shapeshift alternate druid class feature from the v.3.5 Player's Handbook II) as heavily as it has, and I really like they way they did it. Now, not only is it balanced, but you have oh so much less rules referencing to do, even more so if the polymorpher in question has neglected to do their homework beforehand and decides to learn the polymorph rules mid-combat.
1. Simplified/Unified Combat Maneuver System: I've run two vampire-heavy campaigns and been on both sides of the screen for PC monks, and there are few rules more annoying than those now collectively grouped under "combat maneuvers." Although this section goes to the benefit of multiple special attacks (disarm, trip, etc.), I'm going to focus on the historically most grievous offender: Grappling. I won't claim it's simple and easy now, because it's still pretty complicated, but it's much more streamlined. The unnecessary parts have been cut, what remains is better defined, and the whole thing seems much more manageable.