"I know it when I see it"
Erica used to insist to me that I had to have "a type", and would get quite agitated when I would insist that I didn't, really; I just went by the philosophy espoused in the above quote (yes, the reference is intentional, as if you had to ask). I mean, seriously, I pose this as a challenge: see if you can find any commonality among my last three crushes (assuming that you know who they are/were, of course). I can only come up with two: they're female, and they're all younger than me. The former is kind of a prerequisite, and the latter is incidental, and neither is all that useful as a guide to this nebulous concept of "type". If you can come up with anything better, I'd be most impressed. Heck, you could probably shorten that list of three to two and still fail to come up with any more useful common trait than the two I just mentioned.
I feel the same way about music sometimes; less so, since I think it's easier to say "these two bands are similar" than "these two people are similar", but sometimes I'm surprised by just how much difficulty I find in articulating what I like in music. It's partially that I lack the vocabulary, I think, but there really isn't that much of a common thread -- at least, not one that I can really see.
The problem with this viewpoint, of course (staying on the music side of the example) is that it's harder to determine whether I'll like a given band or CD or song that I haven't heard before, which tends to discourage my exploration. I'm sure you can complete the other half of this analogy.
[Update: Kenneth points out that another common trait is intelligence. I suppose this is true; I think of it as pretty much of a sine qua non, although I might make an exception for, say, Tyra Banks. But even as I write these words, I know that that's not really true, either.]