I know they're missing features. Steam didn't always have all the features they have now either. We take that for granted now, but that wasn't always the case. Steam has their market position because they were one of the first to market, and they invested their profits into making their product better. A newcomer is now met by consumers with the expectation that they'll be equally as good, and when they inevitably aren't they don't use it. That's how a monopoly works.
I agree it sucks, and that Steam is a nice piece of software, and the Valve has done good work supporting Linux. That doesn't change whether or not they hold a monopolistic position in the market though. The barrier for new entries into the market is too high because Steam is good. It's nothing against Steam, but it is a monopoly and that isn't good for consumers —which includes game developers, not just end users.