*looks at 22GB of extra stuff in his Skyrim folder* Yep, I've been known to mod a game or two.
Thief's running on the Unreal 3 engine and I've not got much experience of modding it - Just Wiki'd the list of Unreal 3 games and I can't remember doing anything much more than graphics mods for any that I've played. It's only out a week so I'll give it a while before hitting ModDB or The Nexus (which now covers pretty much every game made, if someone can upload a mod which is acceptable).
If I'd paid full price for Thief I'd be harder on the game than I am, but even then I'd be having fun with it. I'd wholeheartedly recommend it to fans of the series - The difficulty aspect is bunk seeing as there are so many options you can configure - because I've yet to come across something which
shouldn't be there. It's rough around the edges though, as I've mentioned, and weird for something which has been so long in development - Hopefully that doesn't signal Square getting tired with the developers and telling them to release the game or else. If a studio's desperate to cover its losses we mightn't see the patches the game requires.
I'd say you'll need a pretty good PC to run it well - For giggles I installed it on the old PC (Phenom II/560ti/8GB RAM) and was getting 30fps in the benchmark (EDIT - Tried actually playing the game too and was getting lots of stutters, even indoors) on Very Low settings. That PC can run Sleeping Dogs with the HD Texture pack at 60fps all day long on Med/High settings and my new PC runs Thief at 70+ on Very High. Poorly optimised, but there are a few workarounds to help things out and Unreal games have a lot of performance options hidden from normal view in the ini files.
I bought it for 50% off as a pre-order so discounts are out there. I use
IsThereAnyDeal to hunt them down nowadays, it seems the most comprehensive aggregator right now.
And to your final thought: Thief gives you a burst of frankly terrifying techno music if you get caught.
It's not as offputting as it sounds but the levels are far too loud.