You should always takes notes on players, not only for future reference but also to help you play against them as you are playing. I find this helps me concentrate more on the tables as I'm playing and reminds me of which player is which... Yes I'm a bit dense.
You'll also be surprised of how many people you will come across in future times... I've often found a note on someone that I played a few months prior and have been able to play them appropriately from the get go.
Just as guide for taking notes; look out for how people play certain hands and if they are playing them in an unorthodox manner. For example, most players don't tend to lead out into the pre-flop raiser with a set, if they do that's something to take note of. Conversely if a player leads weakly and folds a lot to re-raise, I would note that.
Things that I generally don't note are if a player is loose or tight, or if they raise pre-flop alot... Basically anything that my
Poker Tracker stats will tell me. If you haven't got that I would highly recommend that you start using it too.