I think in one word: intent. Is there love behind the judging or concern? Also, I think it's important to remember that as Christians we are called to correct others if what they're doing is sinful, but it must always be done with love. (Which is hard and I fall short on this one sometimes myself ;) Just my 2 cents!
 

When it comes to being judgemental, I always thought of it as whether or not you are stating that and action is right/wrong or if you are talking about the heart of a person. For example, you can say to a girl who's had an aboriton that what she did was wrong, but we can't say she's a bad person, or an evil person because we don't know her heart. Did she have free will? Did she know of her options? Did she know it was a sin? etc. etc. So, I could say saomething is a sin and it not be judgmental, but I could never say someone was bad or evil in and of themselves.

Some people say that when we discriminate over who we socialize with, then that is being judgmental. I think that is a fine line. Some people believe that we should distance ourselves from those who are living sinful lives in a blatant way. I know some people who have stopped socializing with people who have come out as homosexual. I know some who even won't be "too close" with those who aren't as holy as they strive to be. I guess God calls different people to discern in different ways. But I think it can be a fine line too easily crossed. What is good discernment vs being judgmental?

However, we live in a world where even saying sopmething could possibly be wrong is judgmental. Christians get the brunt of this. If I even utter to someone that "gossiping is wrong", oh, well I'm being judgmental. It is just a way for people not to have to feel guilty. Guilt isn't always such a bad thing.

Anywas, those are my ramblings on the subject.