The vendor-exclusive content bothers me. Steamworks isn't inherently bad - I've a few games that use it - but it has some real problems. For example, the last game I purchased that used Steamworks (Supreme Commander 2) did this:
1) Valve let Steam pre-orders pre-load the download installer so the game can be installed as soon as the release date/time hits. Great! So, I pre-ordered through Steam and pre-loaded.
2) On launch day, Valve forgot to decrypt the pre-loads. So I had a couple of GB that couldn't be installed. Morons.
3) While that decryption comedy played out, GPG releases a launch-day patch that modifies a few of the compressed archives for the game. Requiring a full download of each of those archives because Steam can't handle differentials (at least not that day). So the pre-load, decrypted 13 hours later, was worthless as we had to DL another 1.1 GB anyway.
4) If you bought the disk version, you needed to DL the day 1 patch - no way around it. You couldn't even install the older (disk) version of the game because Steamworks doesn't let you do that. So a bunch of Aussies on bandwidth caps killed some kangaroos and koalas out of anger at having to pay a large amount of money to re-re-download the game even though they bought the disk version. They use the DVDs like that feral child in Road Warrior to decapitate dingos. Aside from that, it serves no other purpose. You can't install the game with it.
5) If Steamworks burps, you get kicked from your multiplayer ladder match, as reported on GPG forums. I don't play ranked, so I've not experienced this.
But my biggest on-going issue has to do with vendor-exclusive content, where I paid more for Supreme Commander for the priviledge of buying through Valve's Steam client. Where I could have saved $5 - 10 (USD, depending upon sales tax and shipping options) and received vendor-exclusive maps. And you can't get all the maps unless you live in France or some
where they had a vendor that provided all 5 of those vendor-exclusive maps.
Bonus items really shouldn't be substantial in-game content. Maybe some aesthetic thing that doesn't impart additional content/story/features, but just looks good or something. I hate having to pre-order from Steam to get the Neb talking head and the map. We'll probably see Gamestop get some
different map or something. I hate that crap.