In defense of crossovers - a well written crossover gives enough information about both sets of characters for the reader to make sense of them without prior knowledge.
As a writer I can't assume that everyone knows Frasier Crane, so I have to introduce him as though he was an original character. I can't assume anyone here has heard of Commander Straker and SHADO, so they have to be introduced and the background given (or not, if it's not important to the story).
I can safely assume everyone knows who Lois Lane, Clark Kent and Superman are, but I have to give enough particulars so the reader knows which versions I'm using.
But some crossover possibilities work better than others. Star Trek, Next Gen and Voyager don't seem to work well with Superman, but it could just be the writing.
And, for whatever reason, Superman doesn't seem to generate all that many crossovers in general (except for Batman - but then they're in the same universe so that's a natural). I've seen X-men, one Doctor Who, X-files has been done and there's an SR/Stargate over at ff.net that looks promising but hasn't been updated in ages and ages. There's an SR/Dexter that's not bad.
There's a Remington Steele one floating around here. I've done UFO and Frasier.
But Superman is one that just doesn't seem to play well with others. If you remove the Batman xovers, there's not a lot out there.
