I typically take notes on the following:

- what their pfr's mean (size wise) sometimes players min raise w/ AA, sometimes they bet 10Xbb w/ AA....
- How much faith they put in TP type hands....did they call large value bets for 3 streets? Did they shove w/ TP 9 kicker? etc...
- Do they slowplay big hands (sets+)....
- Do they call FD's or straight draws with improper odds...
- Do they bet small on flop and turn then bet large on river with TPTK or 2 pair type hands....

Basically anything that allows me to get into their head to see how they are playing...