Long books, usually. Plots I can't guess the ending to at page 35. An ending that feels earned. An antagonist that leaves me seething with rage at the mention of their name for years after (hem hem). So rare, but those books are worth their weight in platinum. If I could find a book or series with all 4, I'd buy a lottery ticket.

I will read almost any book, but whether I finish or enjoy it is the question. I finished a book the other day that introduced a character in the climax that was there under another name/in another form from the threshold-cross, and her actions ended the conflict, just by being there. It was not a good pay-off.