It means you can't resell the game, even if you have a physical copy. That's extremely limiting for those of us who like to make some money back after we finish a game, or like trade games, etc.. I get that you can't resell a digital copy (although there is Stardock's GOO system), but that shouldn't be the case for retail boxed versions. This is my main complaint about Steam "protected" games. The service itself is fine and works well otherwise. I will still buy games that I like eventually, but never for full price since they take away my first-sale doctrine rights.
http://en.wikipedia.org/wiki/First-sale_doctrine
Because of these restrictions i've started to lean toward consoles a bit more since they have a very thriving used market. It's too bad really because a solid used market could greatly help the PC industry. I mean, just look at the console industry as an example of that.
I sell and trade PC games on Craigslist fairly regularly, so there goes your argument right out the window.
These, and add eBay, Half.com, Amazon Marketplace, and Goozex to the mention of Craigslist. Not to mention maybe handing it down to a nephew or little brother when you're done - completely impossible with Steamworks. It's not a publisher's place (or anyone else's) to decide
for me what I can do with a physical copy of media that I've legally purchased (game, book, movie, whatever). I've bought non-transferable digital games, but only if the sale price makes it too good to pass up ($5 for Riddick: Dark Athena, $5 for Torchlight), but to have the same degree of non-transferability (read: total)
forced upon a physical copy bought in a store is an affront to consumer rights codified into law.
I was very interested in AvP3 and somewhat interested in MW2 until I found out about the Steamworks nonsense - no sale.
I would personally never buy a used PC game. It would be extremely easy for the person you are buying it from to download a pirated copy of the game and then use the CD key that they purchased to continue playing it, giving you a useless game if you intend to play it online.
That's where doing one's own due diligence comes in (advisable in all things, really) instead of guessing. In the case of some titles that's true, but not for all. It's
caveat emptor, certainly, but believe me, the used PC game market is absolutely healthy, aside from those titles where the publisher has intentionally (and, IMO and that of the courts so far, illegally) blocked it.