I hate steam they want to put this crap all over an screw that I have when you have to run steam to run your game.. HATE THAT! Direct to drive, download an play.. ahhhh No crappy program in my way. "Sleeps happy!"
Steam is great, it's much better than D2D as far as a digital download service. You have all your games for life, and all you have to do is log into your steam account on any computer and it will automatically download your entire library for you. Plus the community aspects, friends, gifting, really REALLY awesome deals (they do weekend deals like this all of the time, and free-weekends), preloading games that haven't come out yet so you get them "day 1", etc.... it's easily the best digital distribution method out there by far IMO, they really do it right.
D2D is just a raw download, and they only give you the download once for life. No redownloading, no automatic updates (which you can disable with Steam), and frequently there have been installation/game patch incompatability issues with D2D versions of games, etc.
If you're not a fan of having a unified games platform to run some of your games on then you're better off just getting a boxed set from Amazon or a local store, which is what I sometimes do. Atleast then you can have a physical copy for life and be able to use it as much as you need to between computers etc (most of the time)