While I like almost all of the changes, subtle and not so subtle, for me the best part of Pathfinder isn't a specific. The folks at Paizo did an excellent job of refreshing one of my favorite pasttimes, breathing new life into it. It feels shiny and new, and I've got some of my old childlike sense of wonder when I read it. This is also true of the Golarion campaign setting - very exciting stuff!
That said, some specifics that stand out for me:
Each core race now has much to recommend it - at some point I'd actually like to play a character of every one of them because it should prove unique. This is especially true as I've gotten more into the Golarion campaign setting, where even more background and culture has become available to draw from.
Similarly, the overhaul the core classes got is fantastic! Every class is now eminently playable all the way through level 20. IMHO, this restores Prestige Classes to their rightful place - alternatives taken for flavor or a different experience rather than as a necessity to make starting in some of the core classes competitive at higher levels. Also, each of the core classes now has a very unique feel. The most blatant example of this last (for me) is the Sorcerer and her Bloodline powers, but every one of them has similar enhancements that could be cited.
Finally, the overhaul and simplification of the Skill System, eliminating the need for a calculator and an advanced mathmatics degree to correctly allocate skill points (especially for multi-classed characters with different Class Skills).
There's a lot of other stuff I like, and (obviously) very little I don't like. And the stuff I don't like tends to be stuff I can easily house rule - as intended by the designers.