Lots of little things. But the only real Thief game I can actually say for certain that I remembered playing was Thief 3. The things that bothered me were some archaic technical limitations of the era, but they were irksome nonetheless.
You have to understand though, that I'm somewhat of a graphics whore so a lot of my complaints are about the aesthetics. Little visual flaws like your character model clipping through some environmental structures. When you picked up crates and other items, the object disappeared from view. Character animations were a little stiff. The 3D models were not as believable as I would've liked them to be. The HUD and user interface could be streamlined a little bit to be slightly more intuitive.
It was just one of those unfortunate games that came out during a transitional period of gaming technical advancement. The games that I find the most appealing are those that come at the peak of each age of innovation. Like Doom was an evolution of what Wolfenstein 3-D, the former of which I consider to be more of a classic than the latter. Then you have Half-life which piggy backed off the trials of the Quake era. God of War which came towards the end of the PS2's life cycle.
My point being that Thief could use a current gen update with all the shiny new normal mapping, advance real time lighting, Havok physics, and full HD resolution glory. Despite what people might tell you otherwise, pretty graphics matter a lot in contributing to the entertainment value and immersion factor.

I really hope the Thief series comes to GoG. I'd love to have the Gold version of the first game, that and I've had nothing but trouble getting the first one to run on my current rig.

Ah it's Thief 2 - in that case you don't even need to optimise.
I have no idea why DDFix is failing to patch. I guess you could try the manual method but it's a bit of a pain. I think I also saw pre-patched .exes somewhere.
Sorry I can't help much more than that.

I was told that it's because of safedisc, so I tried some cracked .exe's from the usual places. When DDfix didn't work even on those I just gave up. I'll give it a go again later.
Thanks anyway. Always a pleasure to meet a fellow taffer.

Heh heh heh - Go on, taff off.
I want to see Taffer, Taff and Taffing in the dictionary.

WTF is a taff? Or do I want to know?
If it is what I think it is, please tell me more.

Taffer = probably means anything from "asshole" as in "Hey asshole, come on out!" or scum "Come on out, we know you're in there, scum!"

A Taff is a person who wields a Dessert Eagle

http://www.urbandictionary.com/define.php?term=Taffer
For immersion, they wanted Guards and such to be able to curse when appropriate, but they didn't want to make it offensive for it's own sake. So the writers in yet another stroke of brilliance made up their own swear word. The writing in the Thief series kicks ass. I don't know where they got that etymology from, though.
I don't know why they wanted to avoid swearing because the game was rated M anyway, but I'm glad they made up taffer. It's like a signature of the series.

Also, it will give GOG a hard time when trying to make Twitter clues for it if/when they come close to getting it.
"Well, we can't use taffer because everyone and their mother's dog will know immediately what game we are talking about."

Well - got it today, installed fine and DDFix'd and all that. It runs, yay.
Only problem is briefings aren't playing, weird.

Have you installed the right video codecs? On "Vista", this isn't guaranteed to work. I had to register those codecs manually.

Yeah, video briefings were weird for me even back in XP.

Yep - codecs did it.

Just thought I'd try this here... I loved the Thief series, but the 'head bobbing' (the feeling that your character was walking *up* and *down*) after a while gave me motion sickness (hey, I wasn't the only one, believe me!). This wasn't a problem in T3 and even though some people hated T3, I really enjoyed it. Games used to be made this way for a while (HB'ing) and I was glad to see this 'feature' phased out in modern games. I never was able to find a fix for the HB'ing in T2 (some of the older games gave you an option in the 'ini' files to turn it off). I realize the answer is probably NO, but did anyone ever find a solution for this? I never was able to finish this one. Thanks...