I think what OOD meant was that we don't have rigid lists of mortal and venial sins. Saint John the Theologian says that there is deadly (i.e mortal) sin and that sin which is not deadly.
EDIT:
I see Mor Ephrem and I posted the same thing.... sorry about that.
If I were Tom I'd find a spiritual father (or mother) who is willing to devote the time it takes to guide someone in the faith. Or else just find a new parish.
Getting back on topic, OOD says that the Orthodox Church doesn't have any categories of "mortal" and "venial" sins. But what does this mean practically? Are all sins in all cases equal to each other no matter what? Is the college junior who plagiarises "as bad" as a serial murderer? Surely there is some sort of "hierarchy" of sins; even Saint John seems to admit such in his first epistle. If Orthodoxy doesn't have categories of sins the way the Latins do, then what exactly does the Orthodox Church teach about sin in this respect?
I think that the "gravity" of any sin is something to a great degree subjective to the individual. Hence, seemingly "little things" in the cases of some people, can be for them, "big" sins. Part of this could be their own scruples, but this can also be do the acuity of their conscience or their spiritual advancement.
However, I suppose you can speak in generalities of some things, both because of their perceived "objective" realities (killing someone is objectively more of an injustice than stealing their pen), and because to a degree you can make generalizations about people (in some fundamental ways, we're all the same). I do think though, there is a danger in coming up with "lists of mortal sins", since they tend to create a "checkbook" view of repentence (ok...haven't killed anyone or stolen anything "big"...but I guess it matters little that I constantly steal stationary from work, or indulge in gossip.) It's just too easy to schew people's outlook that way.
Probably the worst aspect of the "checkbook" mentality, is that it totally ignores what will keep people from the Kingdom of God - the underlying condition of their heart. God's love is more powerful than any of our sins - but it is the condition of the heart, which will ultimatly be our ruin (or the gateway to salvation) for a "bad heart" (irregardless of exterior appearances) will not be able to receive such a Love as blessedness. Hence, a "checkbooker" could easily deceive himself into believing he's quite virtuous, because circumstances have not caused/allowed him to do horrid things to others, or frequently commit "serious" sins. All the same though, he could be in bad shape. 
That's why even Saints recognized their unworthiness, for they had the gift of self knowledge, and the wisdom from God (truly a great mercy) to avoid the temptation of complacency.
Seraphim
"...communicants should not be conscious of any grave sin.."
Well, since the term does not seem to be easily definable, I guess this is a good reason why confession SHOULD be as often as possible!
Otherwise, you are like me, you dont know the state of your soul and whether you should be partaking of the Eucharist or not.
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They say that I am bad news. They say "Stay Away."
I think that we should be able to tell the gravity of our sins. This does depend on our progress, but perhaps we should not be going to Communion unless we know ourselves well enough to tell what is grave and what is not so grave.
We should be watchful and sensitive to the effect that each of our actions has on our inner state, otherwise we are not really living the Orthodox way. St Theophan says that if we have lost the warmth in our heart that comes from real prayer, then we have moved at least half way from God. If we have never learned the difference between the warmth and the cold of our hearts, the difference between the rememberence of God and the ignorence of Him, then, forgive me, we have not learned to pray in an Orthodox way yet, but only in an abstract or emotional way.
The ideal thing is to confess our sins as they occur, and be concious of our forgiveness at the time, confession is only part of the story. We may be concious that we have commited grave sin, but we should also be concious that we have been forgiven before we go to communion. Is this right? Isnt the way we apraoch Communion these days a little bit Catholic style? It isnt a legalistic matter, where we just reel tham off and then are forgiven, it has to do with our state, and this is an ongoing, 24/7 concern for an Orthodox Christian. Does anyone else think this?
We know that heresy is to live a false spiritual life, and I suspect that many Orthodox are in this position because their preists are not able to teach them the Orthodox way. The Holy Communion works in us depending on our state, in a purifying way, or in an illuminating way and we need to be concious of what stage we are at before we approach it, otherwise it will be to our punishment and we will not even know it.
