I think if you were to sort through the old testament, the question of what is an idol is pretty clearly defined as an actual graven image, and there are plenty of passages that reference the corruptibility of the actual idol and explain why it cannot be as powerful as God. I don't want to turn this into a theological discussion and start citing passages, but I think if you limited the question to what the god of the old testament was forbidding, it would be actual carvings and such.
Of course, few limit their reading of the bible to the literal, and the question of idolatry then becomes a far more interesting topic. Even in a non-religious context, a prohibition against idol worship would make sense. The worship of money, success, fame, admiration, etc. would all be idol worship because it seeks something other than that which ought be worshiped. In a religious context that which ought be worshipped is the true god who is actually God, but in a non-religious context, god might be service to others, compassion, or other such things.
So, idol worship is to pray to that which is not most holy, and what is most holy I suppose can be debated. I would say, though, that someone who accepts the concept of the holy and the sacred, even should they apply it in a non-religious way, will present and behave as do the religious because it is doubtful that a thinking person would find holiness in the mundane.
And that takes us to the first commandment, which is whatever it is that you hold to be the most sacred is the only thing you are to consider god. I do think 1 and 2 are wrapped up together. Again, though, if you read the Old Testament literally, you may not accept this interpretation because in Exodus, as you may recall, the Egyptians received the 10 plagues as proof of the supremacy of God (Yahweh) as being more powerful than the Egyptian gods. That is, it is clear there were many gods, it's just that the God of Israel was the strongest and for that reason God (Yahweh) was to be accepted as the baddest of all the gods. Again, though, I don't think the literal interpretations yield as meaningful results as the interpretative.