I think that much of the anti-NB talk you see is a reaction to the fact that for whatever reason, Fat Tire is very popular with 'the masses'. My in-laws love Fat Tire, but as far as I know they don't really drink anything else not produced by BMC. A lot of people who have not tried a large variety of beer think that Fat Tire is the greatest beer ever. In fact, everyone I talked to about it before I tried it thought so. Needless to say, I was disappointed when I discovered that it is (IMO, and in the opinion of most people I've found with more beer experience) an average at best amber ale.
I think part of it too is that NB does make some legitimately good beers, they just aren't as widely distributed as Fat Tire. I haven't had the opportunity to try too many, but I don't think I've ever heard anyone say bad things about their sour ales. Some of their other beers seem to have a 'love it or meh' reaction (Ranger IPA).
Most (if not all) of the criticism you'll see will use the word 'overrated', and I think that's mostly because of the people I mentioned earlier that overhype Fat Tire which gives the rest of us too high of expectations.
EDIT: Upon re-reading this, I feel the need to take a position. I think that it's unfair that NB gets such a bad rap. But olllllo said it better than me anyways.