Legalism is quite a charge. When someone is imposing a legalistic vision upon others, they are saying that unless they do certain things, they are out of God’s salvific favor. Christ + something = justification. So when a person is charged with this, it is serious business. You’re stating that they are preaching a false gospel. Galatians is a case study.
Many times, however, believers who are like-minded on many core, essential things, yet butt heads, sometimes vigorously, over what Christians should or shouldn’t be doing as a result of their salvation, lay this charge of legalism against the other. In all fairness, legalism is probably not always the right term to use. You can usually discern what they’re trying to get at when using the term, overstated though it may be, but legalism is a high charge and doesn’t necessarily fit. The problem though is that there is some truth to the charge, but not exactly in the same way. It needs some redefining.
Whereas legalism puts the “offending” persons’ relation to God in question, legalism light puts the offending persons’ relation to the community in question. In other words, if you don’t do X, well, this isn’t the place for you. Or, since we’re with these people now, doing X, we can’t hang out together. It may not be so overt, just implicit in action. I have a hard time seeing how this squares with what I’ve been reading from Paul on unity among believers in 1 Cor 1:10-17 and 1 Cor 3:1-15.