I think a good, overly-simplified way to describe the differences in regards to legalism in the RC and EO Churches is that in the RC Church, God is seen as more of a judge, and if you do something bad--like skip Church or break fasting etc--the RC Church emphasizes the guilt of the action you committed, whereas in Orthodoxy, God is more of a doctor, and if you do something bad--like the aforementioned things--there is more of an emphasis on the condition of your soul rejecting God's medicine, opposed to the guilt. We don't see these things as just rules God gives out to us for kicks that will either anger Him or appease Him, but treatments God has prescribed to us so that He can heal us.