I can tell you that it would be very hard for Microsoft to make me choose to use their app store over Steam without resorting to anti-competitive tactics. Steam, from a gamer's perspective, does almost everything right, and I have a huge number of games "stored" there. Then again, laziness might trump for most people.
I don't understand that perspective, and I honestly think there is a bit of a double standard going on here. People hate on Origin and 'meh' the rest of the competition, but are in love with Steam. I know people who will only buy games if they're on Steam, it's bizarre.
Your games are locked into steam, and they may never be removed from there. You own none of them. The only way to play them is to open Steam (presuming it's up / not horribly busy / you have an internet connection^) and be shown their market place. There is no way to 'export' your 'purchases'.
Contrast that with other sites where you buy the game and get to download an actual installer, which you can do whatever you like with.
Steam could do a lot better for users, but by locking people into their service they do a lot better for themselves.
^There is an offline mode, yes. Except you have to put it into offline mode while you still have an internet connection, which sucks (what if your internet goes down due to a power cut / accident / misconfiguration at the ISPs end?), and it's not supported by all games.
It's a convenient library of games on an application built specifically for that purpose. It keeps games updated and doesn't get angry about mods, it's also relatively easy to tell which games you have played and which you haven't. It also has superior sales to other places (such as Origin). People like having everything easy to find and in one place. Amazon's service is superior (in terms of DRM, sales, etc), but it's almost impossible to tell what games you have played/completed and the UI is not nearly as pretty. It looks like they took the order list and tried to update it for digital downloads.
Basically, Steam knows what product they are selling, they sell it cheaply, they sell more of it in one place than other (like GoG and GMG), they let you mount games that aren't Steam games into it to help keep it organized. Other services simply haven't caught up and gamers seem willing to accept limitation in exchange for convenience. It's an easy target for disruption, but no one has managed to yet, Amazon is working hard but their UI is not up to snuff.
Also, achievements, friends, and groups are a nice touch that doesn't exist elsewhere in the same quality or quantity.
Don't forget the friends list, in-game/cross-game chat, and growl-style notifications when your friends start playing something so you can hop in a game with them. All those other things you mentioned are huge as well, and I just don't see Microsoft replicating all of these things well. Cross platform is also nice, where applicable. Having to be online and have steam running before playing is the only major complaint I have.
In addition to the other posts, Valve has built up a solid amount of good will with the gaming crowd. They might not have the best support, but their actions up until this point suggest they take gaming seriously and just don't see all their users as a revenue stream, like you see with EA and Microsoft. They aren't beholden to shareholders so as long as guys like Gabe are around, people are less worried that Valve will start fucking with customers in the way you see other companies do.
Installers are an annoyance when game patches are released. Steam makes game updates seamless.
Right now, the benefits of Steam far outweigh the fact that your games can't be exported. I guess time will tell if that will change. It's not like Steam has always been the darling of gamers.
Some games have license keys, which work if you do a manual install. Some games also work fine outside of steam if you activate the game executable from the filesystem (all of the admittedly few I've tried have worked this way)
