My biggest turn off... a lack of tension.

This may be silly, but the one thing most likely to stop me from reading a story is if nothing actually happens.

For example, two people are in love, they get together, they angst a bit, but without anything actually in the way of them getting together, and then they sleep together, promise eternal love, end of story....

I've seen a few fics like that and it's booooooring as hell.