"How is it different than Steam, other than speculating that Microsoft would never aggressively discount games as much as Steam does now (even though it took them several years for their store to be well-liked by gamers)?"
The main difference is that Steam is an OPTION, not the ONLY WAY.
I can get most of the games on Steam off the shelf, either used or new, as a physical copy, or from another digital source (even indie games have most of the time the option of buying a digital copy from their site).
It's a forced comparison: there are ALREADY services, both for xbox and PS3 that do exactly what Steam does, that is to say, provide games as a digital downloads, as an alternative to pysical copies. Steam makes it very convenient to use their service, but it's not something that is forced on you.
What Microsoft was planning was removing any choice. If they come up with a similar service that is convenient and with actual benefits respect to the normal way of doing things, people will start using it by themselves, without it being forced on them.
Only if I do not wan to buy certain games. Two games I bought in a big box store REQUIRE a Steam account to play. Perhaps you have heard of the Civilization series?
More than once when Steam had a problem or I had one with my provider I could not play certain games off line. I don't like Steam, I do not like the idea I have to have their permission to play a game I paid fifty bucks for and have the physical media for. If I had known it would not let me play without an internet connection I would never have bought it. When the label stated "Requires an Internet connection" I figured it was for multiplayer only.