It's not the only thing I enjoy in fiction, of course. But it's probably the primary thing. Captivating characters and decent prose help an awful lot, but I crave the new, and if I remember everything about a work of fiction then I lose the feeling of suspense, of holding my breath because I don't know who lives, who dies, etc.
(This isn't true in a few cases: Comedies where the jokes still make me laugh. Musicals where I like the songs.)
Anyway, I was wondering how other people feel about this:
How important is the "What happens next?" feeling to you, when experiencing fiction?
Note: Please keep discussions general in the comments, specific examples of how it blew you away when you discovered that Buffy's Mum had been a vampire for two seasons at the end of Season 7* are probably not needed.