Too much sitting around waiting.
I've been called twice in the last year and a half. Both times for DWI cases (that's the only thing people seem to get busted for around here). Suffice it to say given my attitude towards DWI laws, I don't get past voir dire. Aside from that, the prosecutor will always ask how you to rate the local police on a scale of 1-10, and depending on how generous I'm feeling, I usually go with 0-2. You can actually see them draw a big X across my square.
The last one I was at was actually pretty damned interesting, though. The defense council was a specialized DWI attorney, and was fascinating to watch. I thought he tipped his hand a bit too much, but he nevertheless did a fine job of creating doubt in every aspect of the system. His client was guilty as all fuck, but I thought he stood a pretty good chance of getting him off. Unfortunately I was swamped at work, or I would have loved to stick around and watch the trial after they cut me loose.