To repeat two of the above:
I like it when it feels like the plot has actually progressed forward.
I like it when something that has a very good chance of working does in fact work.
My own answer:
I like getting away with a creative use of a spell.
I like figuring out a clue that no one thought of (Let's check for secret writing, revealed by moonlight. It worked! YES!).
I love it when I come up with a plan, execute it, and it works. The James Bond infiltration of the enemy fortress, or a pre-planned tactical situation where 'you cast this, you move there, and you wait for the signal' pays off.
I love it when interesting, exciting stuff happens in combat. I'm the guy bullrushes enemies out of windows when he's attacked in his bedroom, jumps on tables and picks up steak knives when he's disarmed, and asks "Can I throw the wizard's summoned badger at the enemy?"
Finally: I love it when I say or do something hilarious and yet appropriate. Like giving a speech to terrified townsfolk, or con a guard, or interrogate a prisoner, and it has everyone in stitches and also clapping at the same time.