I haven't deleted many characters at all-- when I first started playing I was on Cenarius, and I still have about nine alts all there just sitting around, never realizing that I probably won't be back. Now, I'm on Thunderhorn, and pretty much all the characters I've started I've kept. Oh, there's been a few level ones that I created for fun, and I believe I made it to level 12 or so with a dwarf Hunter before I realized I didn't ever want to play Alliance again, but I think that's it-- I just can't bring myself to throw away any characters I actually spent time on.
But looking at the thread, I'm probably the exception. Lots of players toss their maxed-out characters all the time, either because they're frustrated with a class change, need extra room, or are just leaving the game for good.
So what's the highest character you've deleted and why?