I'll give some thoughts.
1. Biblically speaking, Legalism is not a word in scripture. It is an english word to describe something that is, I believe warned against in scripture. Personally, I would consider 2 things legalism:
a. Adding a requirement of some kind of obedience to faith in Christ, that is necessary for salvation. This is what Paul battled the judaizers about: Believe in jesus, AND keep dietary laws....believe in Jesus AND be circumcized. I don't see a lot of this one these days...more of the next one:
b. Adding restrictions on Christians that are standards for obedience, that scripture does not give. This one is much more common, and it is not just fundamentalists that have problems with it. It is basically saying that something is a sin that scripture does not say is a sin.
2. Just for fun, I'll give my thoughts on a few issues...starting with more obvious examples, then moving to some that are more controversial:
-Some have said that Christians should follow OT Dietary laws (Joel Osteen, of all people, has advocated this).
-Some have said christians should not play with playing cards since they have been used for gambling, and might tempt you to gamble.
-Some have said Christians should not celebrate Christmas, since scripture does not tell us to do so.
-Some have said that to really follow God you must live in a monestary, deny yourself all earthly pleasures, and inflict pain on you body.
-Some say that a woman should not wear pants. (though in many cases, pants can be more modest than some skirts).
-Some have said you should not attend a movie theatre (but renting the same movie for home viewing is ok).
-Some say you should not enter a Bar, since you are supposed to avoid all appearances of evil (though I'm convinced the best translation is "avoid every form/kind of evil.")
-Some have lists of musical instruments they believe to be off-limits...with no scriptural support.
Scripture that warns me the above rules are bad ideas:
Col. 3:20 If with Christ you died to the elemental spirits of the world, why, as if you were still alive in the world, do you submit to regulations—21 “Do not handle, Do not taste, Do not touch” 22 (referring to things that all perish as they are used)—according to human precepts and teachings? 23 These have indeed an appearance of wisdom in promoting self-made religion and asceticism and severity to the body, but they are of no value in stopping the indulgence of the flesh.
This verse is of value in warning us against the desire to give the APPEARANCE of religion...attested to by the fact that the amish, with their drab dress, and strict rules...STILL have problems with immorality and drunkenness, and power struggles...because they are fallen humans like the rest of us...
...A church in which every pastor wears a tie, every woman wears long skirts, every hymn is a capella, those things will not stop the people from sinning.
1 Tim. 4:4 For everything created by God is good, and nothing is to be rejected if it is received with thanksgiving, 5 for it is made holy by the word of God and prayer.
(other scriptures allow freedom when deciding on utilizing things/food that has been used for idolatrous purposes...one eats, another doesn't...according to his own conscience...which I think is applicable today to the questions of association: should you shop at JC penney, knowing they support Gay marriage...one says the won't...another says, "my son need jeans for school, we're short on cash, and I have a JC Penny gift card.")
NOW, there are principles that apply to non-biblically addressed issues, such as do all to the glory of God...BUT...I see legalism as applying statements like "avoid the appearance of evil." to mean, "don't do anything that ANYONE thinks might even appear to be sin." If that were the case, we could not do anything at all.
My arguments are Sufficiency of scripture arguments...God gave us what he wanted us to know...if He didn't want us to use guitars, he would have said so.