This is speculation built on top of more speculation, but I'm mostly thinking of the possible official high res texture pack, if it's only available through steam, your suggested advanced buttons aren't going to help.
Ok, I think perhaps you misunderstood me. I simply meant that mods that require code injection or large-scale file replacement wouldn't be compatible with the simple archive/esp system that it appears Steamworks will use. So all mods like that will
have to be put on other download sites. Yes, that's speculation, and perhaps I should have hedged it round with 'I thinks' and 'it looks to me like', but life's short

. As for official DLC, like HD texture packs or new areas/factions/whatever, then the same situation will apply as before. You get it by whatever means as a combination of new executable, new .esm and new archives, and mods will either be compatible with it or not. Ok, not all official DLC will need a new executable, but many will.
It's been clear all along that Steam Workshop will be for
mods. Bethesda may use this method for distributing official DLC, but I doubt it.
I forgot about that statement, it sort of makes it sound like maybe Bethesda has created some new system for packaging and installing mods, so maybe there's nothing to worry about. It'll be interesting to see how it all works.
Well... tweaked at least

. I don't think they've re-done much, and it'll still be the basic combination of assets and .esp - just one of those assets will be an ini file, in addition to the meshes, sounds, textures etc.