I'm actually more inclined to offer the obscure fandoms than the more common ones. That is, there may well be a lot fewer people who feel competent to beta read, say, Mrs Piggle-Wiggle than a Diana Wynne Jones novel or a Georgette Heyer novel.

I know, too, that there are fandoms like the Chronicles of Amber that always get fics written but that nobody ever offers to beta. I'm not sure about offering that, though, because I requested it. I think it would frustrating to discover that the only person willing to beta one's fandom was the recipient. Then again, there were seven requests and at least six offers, so...