These are the games I have on Steam.
I have about the same again in physical games, once I decided I didn't want to use Steam if I didn't have to. Basically not using Steam means I can sell them if I want to. Buying from Steam means the games always belong to Steam. If they want to cancel your account, they can.
I had a dispute with them once, when a payment I made to them was messed up by Paypal. I created a dispute, because they didn't give me the game I paid for. They cancelled my account, together with over £1000 worth of games in it, and there was nothing I could do. I had to wait until they realised the problem was at their end before I got my games back.
Read the user agreement. It is not good.