Do you mean having a theme is restrictive? I think it makes it easier to judge. It's easier to pick the best panning shot (and everybody's got one of those) - than to try and judge an excellent panning shot against an equally excellent arty shot - they're two completely different things. It all becomes much more subjective - perhaps more people like a panning shot than an arty shot - but it doesn't mean that the arty shot isn't good. Still, I'm easy - so long as we get somewhere with it.
How about a competition to see how long it can take a bunch of photographers to organise a simple photo comp?