Let me preface this--I always feel the same way about fantasy defenses as I do about kickers. That is, it's often hard to predict and scores can swing wildly from week to week. But if you have a good one, the best part is usually that you have some semblance of consistency at the position. This is also a position in which match ups can be important. For instance, if you have a floundering D/ST mid-season and they have a bye coming up, you can often find a mediocre D that might be going up against a young QB who is prone to interceptions (and maybe pick-6's).
Here are a few other thoughts on defenses in 2012:
New York Giants--well duh. But still,