I just gave several good reasons.
How about I break it down and make it simple for you:
As a system:
1. It demands you run the client in order to play games, even if you're in offline mode.
2. Said client tends to screw up regularly, i.e. the "Download starting" issue
3. You can't remove the community crap
4. In order to activate offline mode, you need to be online in the first place.
5. And in order to continue to use offline mode, you need to connect once every four weeks or it stops working.
6. You usually can't use the retail patches on the Steam version, and it can take Valve weeks to get around to pushing out a patch, if indeed they ever do, even if the developer has provided the patch prior to the retail version
7. It sends information on my use of the client, as well as the system it's running on, back to Valve
8. It resets settings such as whether you actually want to receive updates for a game, or your download location, on a regular basis
9. If Steam is busy, you can be prevented from playing or installing a game because the servers can't take it
As a store:
1. Prices are ridiculously high compared to retail stores, let alone competing digital stores
2. Valve's customer service is non existent
3. Thanks to deeming the service a subscription rather than a store, they can ignore most trading laws
4. They can close or cancel your account at any time and for any reason, including if they think you are cheating on an online game
5. If your account is registered to an address in one country, and you try and activate a game from another country, they'll block it. Complain and they'll close the account as 'fraudulent'
6. They provide the US version of the games, which can be annoying in cases where content is cut (The Witcher, GTA) or where the multiplayer aspect is tied to a specific geo (Sacred 2). Thanks to point 1, you're often paying more than it would cost to import the damn game from the US, let alone buy it in your own country
7. Your dependent on Steam being up and running to continue to play the games you have purchased.
8. No refunds
9. Tech support is absolutely laughable at the best of times, particularly for non-valve products
Like I said, they're the new Microsoft. I've had a Gamer's Gate account for years, they've changed their service in response to criticism and the desires of the users, most recently for example they've ditched the client software and noted if a game includes a DRM system on the purchase page. I've had an account with Stardock since Gal Civ 1 was released, and again I've seen it change in response to the comments and suggestions of it's user base.
I've been with Steam since Dark Messiah, and the only change they've made is a completely useless and redundant community service which is inferior to X Fire. Oh, and they finally got around to showing prices in local currency; something which was "coming soon" back when Half Life 2 was released.
So we're basically down to a company pushing a shoddy and inferior service than their competitors which publishers love because it provides them the kind of control over their product which you simply don't get through traditional methods due to consumers hating it and being unwilling to pay for it if given a free choice. Sounds familiar.
It's not to say Steam is unrelentingly ****, it does have some useful features too. The problem is most of those features have now been incorporated into their competitor's systems. While Paradox, Stardock and even Direct2Drive have consistently tried to improve and innovate in their service to attract customers, Valve have sat on their arse and done sod all. Of course, given they can handcuff you to their service via the publisher there's no reason for them to even try to improve.
Funny thing is, when EA does this kind of **** they're deservedly lambasted for it. But Valve can't put a foot wrong ....