Most of the love came from the community features they added. You can see what all your friends are playing at any time, when they join a server or log on, you get (optionally) a small popup about it in the corner (even while you are in another server, or playing another game.) If you decide you want to play with them, you just hit shift tab, and a small community overlay pops up, where you can join any of your friends that happen to be playing on their server.
You can chat with them (a small popup will appear, also able to be turned off), and steam supports voice comms across games through the same community interface. It gets a lot of use even when you are not necessarily playing, as a way to look and see what your friends are doing… have conversations with them, which, by the way is also supported by voice comms seamlessly.
There are plenty of other ancilliary and nice things, each player has a default page, statistical and achievement tracking, for the various games. There are group functionalites, useful for clans: group pages, shared calendars for upcoming events or matches, chat rooms, your usual social bs. It has some support for games not created with steam in mind, yadda yadda ya.
Note that it also caters to the online gaming crowd, and so some other things are also a big deal: steam includes VAC as an effective anti-cheating and anti-harassment means – the threat of losing your steam account insures that most people are not posting offensive naked pictures on the sites or in-game spraypaints, and they are not risking cheating.
Also, patching takes place in the background, and happens simultaneously – you may not care particularly if single player is your thing, but all clients and servers need to be on the same version… so you’re assured that the next time you want to pop in for a quick 10 minute ctf session, you won’t instead spent 30 minutes finding that servers have a later version, then hunting down the appropriate patch, possibly waiting in a queue, finally finding out that one of your favorite servers has not updated yet, and so on.
The integrated vcom is good enough to settle issues with which ventrilo version, or teamspeak because ventrilo doesn’t run on linux and so on. In general, Steam largely solved the most persistent and annoying problems of the fps online community.
There is also a good deal of valve love rubbing off, given the support and quality of their own products. In general everything works very well, and while it does take a little time at startup, once its running it doesn’t impact performance.