Bear in mind in understanding this reply, I'm not a theologian, but I'll take a stab at a reply given a layman's knowledge of ecclesial matters. As you indicate, Orthodoxy does not have the legalistic hierarchy of sins, as does the Roman Catholic Church, although saints and Church Fathers have written about sins that are more grievous than others. In Orthodoxy, sinning is a state of separating one self from God. Failure to attend the Divine Liturgy is separating one self from our life in Christ and His Holy Church. I believe there is a canon that essentially excommunicates one for failure to attend the Divine Liturgy for 3 consecutive Sundays, an ancient canon not enforced in our times. I'm not sure if it's a full excommunication or if it is a prohibition from Holy Communion and attendance at the Liturgy from within the Nave, requiring attendace in the Narthex, for a period of time, like 7 years, if I recall correctly. Certainly upon repentance (Greek.: "metanioa," a change of mind), through Holy Confession, the spiritual father of the sinner would determin a proper sanction given the circumstances and a remedial path.