I'll offer my two cents. You could start by giving the generic labels to everyone: loose/tight and passive/aggressive. The first thing I try to take notice of is if someone is generally tight or loose, first by their preflop play (% of pots).

I'll note any unusual plays that they make. You have to be careful with this one, cuz you don't want to make generalizations based on one specific play.

Another important thing I try to pick up on is their betting pattern. Do they sit back and call until they make their hand? Will they raise on the come? How do they play pocket pairs? Are they capable of bluffing? Can they be pushed off a draw? There are certain people who will never raise until they are positive they have the best on the river.

Eh, that's all I can think of for now. Hope I helped a little.