I immediately write "?-?" into any persons notes whom I have played against or watched.

As I get more information on them these ratings change to one of the following:
T-P (Tight - Passive), T-A (Aggressive), L-P (Loose) or L-A
(I also use question marks to denote how confident I am about this read of their play)

I also note whether I think they are super calling stations (CS) or fish, if they are a maniac or if they play at multiple tables (MTP - this warns me if they start taking interest in a hand at your particular table they could very well hold a monster).

Along with these I also take note of pre flop raises, especially if I get to see their hole cards and if a player has proven to slow play very good cards (as another warning).

An example would be:
L-A? (Maniac?)
Raised $4 PF w/ 9To, $6 PF w/ QJ
Has played ~90% of hands