When steam was first introduced the bitching about steam was incredible. Moaning about how it needed an internet connection to work, it was slow, it was crap, it didn't work, friends was pointless, it was bloated blah blah blah....
That's because when it debuted broadband wasn't so common, it was slow, it was crap, it didn't work, friends aren't everyone's cup of tea (I only have one, acquired less than a week ago, and only because I bought a 4pack off him...), and at the time it was pretty bloated to have to load up some additional app just to play a game released in 1998.
And now everybody loves it and spends Hundreds of £ each year on games for it. Quite a marked turn around wouldn't you agree ? Steam is becoming the de facto standard for internet games delivery on the PC. Nobody has anything else nearly as developed / refined / polished as steam is. So much so that rather than develop their own systems, lots of developers just publish straight onto steam. Valve has carved a beautiful niche for themselves.
That's because Steam is much better now

I agree Valve have been very shrewd however, using it as a delivery platform for other publishers has seen it become more than just a niche and crucially they have made use of sales, compilation packs and suchlike to get people using it more. Then it kind of reaches critical mass where people have so many games on Steam using it becomes second nature and they actively prefer buying games on steam than by any other method.
Believe it or not in a funny sort of way I think their combination of sale prices, catalogue of older titles and "allowing laziness" (no need to leave your house/wait for delivery, no need to put disk in the drive etc) has actually helped to combat piracy a little. People who are earning a decent living probably don't begrudge paying 26 quid for the THQ complete pack, they don't even need to get off their arse, they can just suck it down and never need for faff about finding the dvd etc. Just look at what has happened in the music industry, once digital distribution was embraced they actually realised hang on a minute, people are lapping this up, they aren't just into mp3 because it (was) free, they like the idea of online acquisition, slapping thousands of tracks on portable players, being able to get hold of rare stuff, relatively cheap prices etc etc.
Seriously I would love to see sales figures for the steam sales, I reckon publishers worried about piracy could make a killing selling games on the cheap (depending on how much Valve charges them of course).