What is idolatry?

There are some Christian ideas that are easily translatable into secular ideas.  For example, a sin is just something that's morally wrong.  And so when Christians say homosexuality is a sin but they're not judging, nobody is fooled.

Idolatry, on the other hand, is a mystery.  You shall have no other gods before YHWH... why?

I'm not up on my Biblical history, but my understanding is that at the time, polytheism was common, and early Jews believed their god was just one of many.  So the rule about idolatry basically expresses the jealousy and pettiness of their god.  It is also an expression of ethnocentrism--you shall never leave our group or ever adopt practices from other groups.  This law is basically awful and a force for evil.

In many modern interpretations, idolatry is not so much about other gods, but about other "gods".  You're not supposed to hold any idea higher than the one god.  For example, any form of addiction can be described as idolatry since the object of addiction is being held higher than God.  For another example, atheists are frequently described as idolatrous because they're supposedly replacing God with reason or science.

This frankly leads to a very poor understanding of addiction or atheism.  It's a peculiarly Christian-centric worldview, to say that everyone is just like you only sometimes they deviate from the platonic ideal in certain ways.  This is the sort of thing that makes people think it's appropriate for Alcoholics Anonymous to refer to God "as you understand him".  Have you considered that some of us just don't have a God-analogue in our lives?

I also find it strange to have a rule which basically says, the beliefs specific to our group are the most important.  Nothing else in your life should be as important.  This is just more ethnocentrism, and it should not be an explicit value.