Primarily how they play big hands, bluffs, and top pair type hands. Also note the board texture and position. Another good thing to take notes on is what their raises mean. Also note anything unusual they do preflop. Like Jyms said, the notes can be super helpful with sticky spots postflop. People do often take the same lines over and over again.

Here's some typical notes I've made:
isolates in SB with A2o
min 3-bet was AA
3-bet/folded from mp vs utg open
flats T8s in co to mp open
called two streets ip with fd, bluffed 2/3 pot on blank river when checked to
c/r'd Q83r flop then c/f'd turn
with flopped set, c/c'd two streets then c/shoved river
flop c/minraise was two pair
never folding 2nd pair
etc etc