See Scot McKnight's "Introduction" in his commentary on Galatians, for an excellent look at "Legalism Then and Now". These posts cannot do justice to that read due to my own limitations as well as the limitations of blogging.
It must be understood that the legalism that was endangering the very existence of the Galatian churches was not the old "works righteousness" heresy that has been a problem in some times and places. It was, instead, the view that faith in Christ should lead to obedience to the Law of Moses. The fulfillment of the Law by Christ and the resultant life of dependence on the Spirit are left behind and lost, in this error. These Judaizers failed to see that a new covenant brought by Messiah meant an end to the old covenant.
Yesterday, we noted that Bible-reading can be looked at by us as something amounting to the "legalism" that was troubling the church in Paul's time. That is, we look at such a practice as necesary in being acceptable to God and approved by others. This pushes the centrality of Christ and the life in the Spirit to the periphery.
There are other "additions" to Christ and the gospel that Scot points to as ways the Galatian heresy can be alive and well today. "At times experiences become the essence of a relationship with God, rather than trusting in Christ and life in the Spirit." (p 39) Any kind of experience would be included here, none bad in itself (as true of all these activities within this legalism). One might question why you're not doing something or other, which may in its time and place, be a good thing to do. Like some form of protest done in a loving, peaceful manner. Or it may involve a "charismatic" experience. Or some practice of contemplating God.
By any of these, and more, we can unwittingly displace the sufficiency of Jesus, and our dependence on the Spirit. This is very subtle. For example meeting God in special ways can be of great benefit to us. And we can testify to that for the benefit of others. That is good. But one must be very careful to not make such a practice ("personal devotions", special quiet times and practices to meet and commune with God, praying in tongues, etc.) as an evidence that one is more spiritual than believers who don't do such things.
Instead, with regard to experiences, we ought to celebrate the mix that is within the body of Christ, and learn what we can from it, from each other. Instead of dividing believers and churches up as "the haves" and "the have nots".
What Paul was getting at in Galatians, is that in Christ and by the Spirit Christians live and move and have their being. One must be careful not to add anything to that. All experiences and practices that are blessed by God in our lives are derived from the centrality of Christ and the life in the Spirit that belongs to us all. Because of that we can humbly sit at each other's feet, and thank God for the rich diversity he gives, in his callings and blessings, to and through his body/church wherever it's found.