So I read the OP update edit just now and I think it draws a good distinction. It doesn't matter how good one service is compared to the other, as long as there is more than one service. Steam right now is the Giant and Impulse is Jack...
It doesn't really matter where Steam and Impulse stand now, though, because online retailers will boom and bust far faster than most of us think. Youtube is 4 years old this month, for example, and Wikipedia is 8 years old, but many of us have trouble remembering a world without them. Killer apps quickly take over and move through to becoming "the way it is." (When did searching the web become "googling?")
So toward making Impulse the "killer app"
- give users vindication and expression outside of and parallel to the games. Tournaments are an ancient way to do this, social networking is becoming deeper and more integrated, but look at User apps, replays, mods, music, games, videos, machinima, fanfic, and input. What would supporting these look like?
1. A section for each of the above in Impulse with user reviews and cross-referencing by game
2. Easy access to mods in multiplayer game lobbies - if someone else is playing with it you should be able to see and download quickly and without closing the client.
3. An integrated video player
4. User-made games and applications which must meet criteria a la iPhone apps and which are free or cheap to download
5. integrated tool suites for editing and modding files, maps, videos, and games
6. Regular developer recognition and reward of the very best of the above
Many of these things are way too ambitious, but they point toward an application that would transcend DLL and become the default. Build this app and Steam will be knocking on your door, wanting a couple of tickets to the party.