I hate endings where the author destroys their entire universe. I prefer things left open-ended, where I can imagine the lives of these characters going on or not. I don't need to know, necessarily, how every little thing turns out, but wrapping up the important stuff is good, too.

That said, it needs to be, you know, planned out.

I never finished Lost past season three or so because it became readily apparent to me that the writers were just making shit up as they went along. It's clear they never had any idea of what the ending would be, so every mysterious thing that happened, every thing that sort of implied there was a bigger story at work was, essentially, a lie.

Monk kind of ended the same way, which really pissed me off.

I thought the Harry Potter epilogue was superfluous.

In most circumstances, I think it's best when you set up a Chekhov's gun situation. I like endings where I finish the book, stare at the last page for a minute, then flip back to the beginning and go, "Oh!", where reading it the second time enriches the story even further, now that you know what's going on.