One problem with letting too many folks off juries is that you DON't get a good cross section. Then the criminal defendants start challenging the jury pool. That's why they are very reluctant to give any excuse around here, although they'll put you off til the next term . . . once.
Since the Alday murder case in GA, judges tend to grant a venue change if there's even a little evidence to support it. They'll usually bus the venires in from another county.
Aside from what's already been discussed, another discouraging factor for potential jurors is the problem of directed verdicts. Jurors may slave away devotedly through weeks of testimony and many hours of deliberation, only to have all their work thrown out by a directed verdict. The Louise Woodward "nanny" trial is a perfect example of this. Judges take umbrage at jury nullifiers, but don't hesitate to essentially do away with the very heart of the jury system themselves.