I haven't ever tried a crossover in the sense of making a fic with elements/characters from multiple fandoms. I have bene in an (unsuccessful) writing project with multiple writers though using characters from their own fic, but that fell through due to multiple reasons.
I would say that only some combinations of crossovers would work (unless the author was very good at say the crack style of fic for some). Furthermore what would need to be considered is how/why the two universes, so to speak are mixed together, and how characters from one side side would react to the other set of characters/setting/whatnot. How they mesh together would be certainly a main point to consider though imo.
On that note,
I wonder what people would make of some of the potential combinations offer by this site...