Next to all the other good points, Steam has managed not to f up in all these years. If you go deep, their launcher has an insane number of features - trading, backgrounds and picture frames, drops, profile levels and a lot more - but their basic MVP user experience was never cluttered by it. You can right now log into your account from years ago and start playing with very few clicks; if you don't care about all that stuff, it doesn't bother you. The also never really enraged a majority of their users (like digg, for example) and they were the first to adopt a reasonable return policy (it was partly forced, but it's still not standard).

They simply started out well and managed not to loose their lead.