The difference is that Valve has made it quite explicitly clear that whenever their company comes to an end, their games will live past them and their fans will be able to enjoy what they have purchased even if Valve is not there to be able to support them.
Microsoft doesn't even guarantee that they will support their own product after a new generation comes out. Pure digital download without some kind of hard copy and the option for operation independent of the service is just asking for lost product complaints, one way or the other. There is no guarantee that we would be able to have Xbox One games after the Xbox One cloud dies, and some day it -will- die. Games should be able to live as long as people want to play them.
This cloud-calling trigger risks making an entire generation of video games 'lost games' at some point; it doesn't matter if Halo 5 turns out to be a touching drama and becomes the greatest FPS to hit the market in a decade if no one can play it for themselves to prove it, after its cloud support is gone.
DRM's record, so far, is one that has edged into unhelpfulness; pirates seem to have better luck cracking a game than a game with what I'm going to call external DRM for convenience's sake, which in this case is anything that triggers outside of gameplay. This external DRM in turn tends to inconvenience innocent end-users the most (Sony rootkit blues, anyone?). This is what the cloud is, yet amusingly, for all its comparisons to Xbox One's cloud, valid or otherwise, Steam doesn't demand the same constant external call; you can sign in once in offline mode, and be offline for a vast majority of their games, never once connected to the internet and yet still playing.
Internal gameplay DRM like Mirror's Edge and its slowdown mechanic for unlicensed copies, on the other hand, are self-contained, at much less risk to break a system, and usually end up much better-accepted, especially since they've been using in-game DRM tricks since even the NES and SNES age. Cutting the game short or deliberately glitching it because it's pirated is a pretty clever way to waste the time of the pirates, because time and effort is the one thing they can't get back.
Nothing quite like denying a person their accomplishment and letting them know they've wasted their time to frustrate or emotionally crush them.