I noticed that Thief and Thief 2 are not available at this site. Games like, Rainbow Six are but no Thief? Any chance we can get this added or how does this site work? What is the criteria of getting games here?

The criteria is usually the publisher agreeing to it. Square Enix is the owner of the Thief series and is not a signed publisher.

So I just click the big blue vote button and hope one undisclosed day they add it?

That's all we can do. Or contact the rightsholders and convince them to sell it on GOG.

They have to get the publishers to sign a contract with GOG in order to start distributing their games here. If the publisher won't or can't agree to GOG's conditions then we won't get the game (DRM free, single price worldwide, worldwide distribution, fixed price-points).
The wishlist is useless , by now GOG already has a good idea of what games we want here and is waiting until more publishers cave in.

square enix is only marginally worse than EA so I dare not think of the chances of it ever being added.

*sigh* it's companies like square enix that *make* people pirate their games. Not that'd I'd ever do that.

It was on GOG for a few moments, but before anybody could even take notice, somebody stole it.

<.<
>.>
I admit nothing.