impossible to run it on two machines at once. I like to download on one machine (games are routinely 10Gb+) that is not the one I play on. as steam forces you to install in the 1 fixed directory and once you
cant play when offline in 7 out of 10 tries. In theory it should work, but there's always a spanner - bit of account data not cached, steam client update that it knows exists and wont let you play any game until you update (but you're offline), the list is endless. As I am stuck in hotels overnight and do not want to be paying ridiculous internet access rates, it is frustrating.
impossible to allow a visiting nephew to play, say, Plants versus Zombies while you play something else. Even if you "own" 100+ games, you are only allowed to have one machine on at any one time. Cant let one run (say PvZ with the snail on, or a turn based game) and play something else.
Forces spouses who want to try games to buy 2 copies, even if they never want to play at the same time/together
I guess I'm the other way - I don't like GG because of the sporadic DRM that's not really explicitly stated. Sometimes it tells you, but in other cases, you get a nasty surprise. I also don't like the fact that even if you say keep installation files, it deletes the actual installer bootstrapper unless you do some pretty pointless playing around with the file. And the fact of downloading each separately.
I don't experience most of the issues you talk about with steam; you can't specify individually where games should be installed, true, but I have a games drive separate from my OS drive and install everything there, so I guess I never thought about that. The multiple people playing online at the same time is true, but I regularly play offline at the same time on different machines. My daughter only plays certain games- and I didn't want to get her a separate steam account as she didn't really need it. So I bought the game that she wanted, installed it, and she's been offline every since, and I've never had a problem. I also have it installed on my dev machine and my gaming machine, and always keep it offline on the dev machine and only have the problem of multiple logins when I have to update on my dev machine.
I've never liked impulse even though I like everything else Stardock. It just seemed too intrusive, so I was very happy to see Sins on Steam. I wish you could register your current copy with Steam. But I'm happy its there, and hope that the others will be too.