Never played Thief, but I logged a lot of hours in Unreal, 1998. I was (and still am) amazed at how full-featured the software renderer was. I always wished I could peek at the code behind it.

If memory serves, the only thing my 3dfx Voodoo3 could do that software-only mode could not was surface reflections. Maybe something with colored lighting too, it's been a long time. Point is, it was a decent enough substitute for dedicated graphics hardware.