Even if you like the rest of it, the fact that the program rarely seems to work right for a huge number of people should be issue enough. On my own boards there seems to be a running gag with how much crap people have had to take from the program and how laughable Valve's support has been. Several people who used to accuse me of having being on an anti-Valve vendetta have been so soured on Steam after a few months of use that they've more or less taken up the same banner. Not everyone has problems, true, but a lot of people just aren't savvy enough to realize how technically ridiculous Steam can be at times.
But what it ultimately comes down to is that you're allowing more and more power into the hands of people other than yourself. Valve can revoke your right to play its games at any time, whether your supposed infractions are real or imagined, and Steam can deny you the right to simply play a single player game on your own time because it demands that you update to the latest version, even if you don't want to (and yes, there are reasons people might not want to, connection speeds aside). The above poster addressed some of those concerns. And while some of these may be small steps, they're small steps in the wrong direction. Many people don't have a problem with it, and many will call those in ideological opposition to what's being done a lot of unpleasant names, but as things go forward, we're going to have less and less control over what used to be well within our grasp. It's been happening for years now, and it's only getting worse. If systems like Steam continue to grow in popularity, "ownership" will be a thing of the past, and all we'll ever be allowed to do is essentially rent content from owners at their own discretion, though we'll be paying as much or most probably more than we have for years.