I note what I consider to be extraordinary plays. Not necessarily good or bad, but out of the norm.
Does a player raise 4x the blind with 74 off utg & hit a miracle board nobody put him on? Note it.
Also, If I make a particularly bad read and look like a donk, I'll put a note on the aggressive players that they read me as a donk, knowing that I can capitalize on it the next time I catch a monster. (this has made me a great deal of chips)
Basically, any read that will help me, I try and note. Rock, Maniac, Calling Station, ATC. (Am I the last player that doesn't use PokerTracker?)
It's also important to note when a player does something and in what type of game. Just because someone's a maniac in a freeroll, doesn't mean they are in a ring game or SnG, and not noting that can be costly.
Keeping notes can be invaluable, because the lower limit games are very incestuous. If it's 2 A.M., AlaskaGurl is multi-tabling micro-limit SnG's at Doyles Room for the next 4 hours. Ulvid logs on around 9 P.M. and plays 2 SnG's at a time until 3, but he doesn't multi-table as well as she does & is prone to timing out or pushing with middle pair or better.
Hope this helps.