I picked up a habit from watching Eric Clapton... when I play standing up, in order to avoid being perfectly motionless, I keep the beat with my knee--buckiling it slightly to the rhythm. When that leg gets tired, I switch to the other one.
Another Claptonism I have is playing power chords--or DADGAD shapes like 224000--with my index and pinky fingers (instead of the usual index and ring fingers), while my middle finger remains fairly straight.
My wife has also pointed out that the only time I smile while I'm playing is after I've made a silly mistake.
I also have a tendency to beat the living snot out of anyone who heckles me, looks at me funny, or might otherwise be making fun of me. But maybe that's more of a personality quirk than a performance quirk, since I tend to do that regardless of whether or not I'm playing guitar.
I'm joking, of course... but let's not push it, OK?
