What I don't like about Steam is that it makes my games take much longer to load (especially if I have no or slow Internet connectivity), and it leaves itself running when the game is done.  I also don't like the in-game pop-ups it does.  

What I do like about Steam is it makes purchasing games, and managing installs and save games across two computers, easier.  The Workshop is a nice feature, as well.  It also highlights many indie games I otherwise wouldn't have known about (like Fallen Enchantress).  They could use a better browser, but when I'm shopping I just use a regular web browser.

So, Steam has both good and bad points.