-It is a bit slow, but that's because it has to log you in and scan for any updates in all of your games. You can make special shortcuts to make it faster, but even without them, it's not terribly long.
-This is part of the platform, but you could disable autoupdating for all your games and just wait for any updates to download whenever you want to play.
-Just take and copy or RAR up the whole game directory and burn it to as many CDs as you need, or copy it to another computer. Then, reinstall Steam, and copy/extract the files back.
-You can play for three days without authenticating if you don't have an internet connection available; after that, you have to verify via email to extend it by another three days. So, for games that have SP components, only a minimal connection is required.
-Apparently it's not implemented yet. But then, what's the point of it?
-There's some basic stuff that will always auto-update (HL Engine, Steam platform), but if you have a game set to 'Do not automatically update this game', it won't.
However, it looks like the 'You can just go back to 1.5 if you don't like the changes' argument is going to disappear pretty soon. Maybe everyone's forgetting, but WON wasn't exactly the best system either. The in-game server browser for HL-engine games was godawful, and there were fairly frequent outages. Steam's interface is much nicer, it's quite stable, and although it still seems incomplete, it's obviously still getting some love from Valve. Hopefully it'll see even more love when more of the HL2 team is finished, so it'll be pretty feature-complete and robust for The Big Launch.