Legalism—or in plain English: Tiring rules with little-to-no foundation. Most of these rules are unwritten, and some are even good things to follow, though are not "laws" and should not be shoved upon those around us. We as Christians often strive to take God's place unconsciously, or try to put our individual choices upon others and make them follow them—or label them as "bad" or "less spiritual" Christians because the do not play by our rules.
For example: Because it is my personal choice not to drink alcohol does not mean that I have any right to look down on those who do. Nowhere in the Bible does it say "Thou shalt never drink a drop of fermented drink". Christ drank wine, am I better than Him?
I do reserve the right, however, to confront my friend who may have a drinking problem, because we are instructed to refrain from drunkenness (Eph 5:18).
Anywho, it has been so great to read about some of this in my Paul Bible Study. It's sometimes hard to let God be the Boss. To allow Him to make the rules, and not try to make my own. To remember that I am so sinful myself, and that I cannot be better than someone else just because I wore a skirt to church last Sunday while someone else wore jeans.
The Lord does not look at the things people look at. People look at the outward appearance, but the Lord looks at the heart. 1 Samuel 16:7b