For me it's co-op, any game with good co-op structure and a good partner instantly gets a boost in the rating.
Also, the game is just fun to play. I played only the 1st campaign, sure, but it had a great feel to it. Excellent atmosphere, fun levels, and some good tense moments. The shooting and melee is a blast. The game is beautiful to look at.
And the cheesy things like screaming for Dice to open a door while she looks for crates to slide into.
The negative things are that even the few puzzle segments are made moot by waypoint markers. There's some great parts though where you're trying to outrun a deadly fog or need to navigate a maze-ish level with an baddie chasing you.
I would scream like a fool if they made an RE in the vain of RE1 or RE2, but since that's not happening anytime soon I'll make due with this. A fun game packed with stuff.
Plus, it's horror movie mega-cheese
In general I feel that reviews are poor because the game is not what people wanted. I don't think reviewers are being totally objective.