Lots of stuff, actually.
Almost all the conducted performances I see are of contemporary classical chamber music. in these cases, it's not a huge ensemble, and the conductor is there to help the players navigate difficult rhythms together, first and foremost.
I tend to notice if the conductor modifies his/her beat to coax expressive effects out of the ensemble, or if the beat remains precise and unvarying in one hand while the other takes care of shaping. I'll notice how smoothly the conductor manages page turns, whether cues are given with a clear preparatory gesture. Also just the general form and appearance -- some conductors have very "vertical" beat patterns (which I've always hated trying to follow as a performer), some conductors are immobile from the waist down while others (like Leonard Bernstein) dance around a bit. Baton versus bare hands is another obvious variable.
It was fun seeing Claire Heldrich conduct Elliott Carter's Double Concerto once -- there's a notorious passage in which the conductor needs to beat two different meters/tempos simultaneously.
― National Roffle Association (Paul in Santa Cruz), Monday, 19 December 2005 18:36 (twenty years ago)