I play mostly shorthanded limit hold 'em, so my answers may or may not be of relevance to you, but other than some of the things that Chuck mentions:
* How loosely does this player "peel" on the flop (e.g. pay one small bet to see the turn)? Some players fold if they don't have a piece of the flop, some players will call with any two on the flop. And I mean, any two. Since this ties directly into being able to read his hand with some certainty, I find this information highly useful. "peels flp w any 2" is a good note to have on someone, since I can happily valuebet my second pair again on the turn if I know this about someone.
* Big hands - how are they played? Specifically, AA/KK and AK are interesting to know about. Does he limp with AA and KK? Does he continuation bet with AK always? Etc.
* "thinks K-high has SD value", does he go to showdown with k-high, desperately hoping to snap off a bluff?
* "folded to a flop checkraise", when this happens, I don't actually know what hand he held, but since this is such a rare thing to happen, I at least know that he has something when he calls my checkraise.
... etc. Anything out of the ordinary, anything that I don't already know about the player from looking at his PT-stats. AK is important since it's a hand that is always played, and is more common than AA and KK combined - and if you can also find out if he plays AK differently from AA and KK preflop and flop, then you've really hit upon a potential goldmine of information.