I was Web searching "legalism" and came across a conversation between Howard Hendricks and Chuck Swindoll in which Rev. Swindoll defined the problem with legalism like this:
"When we get into areas that are not set forth in Scripture, either in precept or even in principle. These may be such things as length of hair, tattoos and other body piercings, skirts or pants for women, makeup or no makeup. Those are not scriptural issues. Sometimes these issues are cultural, and you do have to address them when you are in that particular culture. But I think legalism begins when you do or refrain from doing what I want you to do or not do because it’s on my list and it’s something that I am uncomfortable with."
Then he gives some advice we all should take seriously:
"The problem with legalists is that not enough people have confronted them and told them to get lost. Those are strong words, but I don’t mess with legalism anymore. I’m 72 years old; what have I got to lose? Seriously, I used to kowtow to legalists, but they’re dangerous. They are grace-killers. They’ll drive off every new Christian you bring to church. They are enemies of the faith. Other than that, I don’t have any opinion! So, if I am trying to force my personal list of no-no’s on you and make you feel guilty if you don’t join me, then I’m out of line and I need to be told that."
You can read the entire interview on Ed Stetzer's blog here.