From what I can remember, Steam was originally a somewhat clunky program. There were days when key generating programs allowed multiple accounts to be activated (and, subsequently, banned for hacking in Counter Strike). There was only support for Windows, and I spent many great hours in LAN games with my friends. Fast forward a few years, and Steam really started getting things together. Instead of having a single key to activate Steam, installation and activation became free, and games were put on sale through the store. Compatibility moved to Macs. Good stuff happened.
Steam has been great for indie game developers. The distribution is simple, and exposure through Steam can help devs get their product seen, and if the game is good, everything else falls into place. Steam has also offered great deals throughout the years, and equally fun prize systems (like the prizes this Christmas). The only way things could get better is some games were free. Which happens occassionally. I just turned on Steam and downloaded a game to play free for a while. I got Portal for free (a great game). I bought most of my games at a thirty percent or more discount. Skyrim, anyone?