DMC games have a very specific "style" to them. Combo based action, ludicrous cutscenes/story, think a punk/emo anime crossed with the Matrix. I've enjoyed a couple but they're not going to appeal to most everyone.
Not a fan of Anger Joe but he's probably a DMC fan so the rating isn't surprising.
And I agree with you, Alpha Protocol was great title that got undeservedly trashed.