The Engine
UT2003, unlike UT, is a fresh product with no basis on the original Unreal engine. Based in part on Canon’s MathEngine, it features some of the most realistic physics ever seen in a game. Every horrible death will see your character thrown, rag-doll style, in all directions. The net is rife with descriptions of incredibly painful-looking death sequences for the game; my own experience involved a very deep chasm with lots of poles to crash into, finishing with a groin-shattering fall directly astride one of said poles. It looked cool, and is certainly not something seen in any other FPS games.
On the subject of graphics, UT2003 features some of the most luscious visuals seen in a game. Every surface is mapped to perfection, with smooth curves and perfect lighting. Grasses swish in the wind. Those with decent graphics cards (i.e. not me) can also switch on full pixel-shading for hardcore water effects.
I want to note, however, that the game was none too stable on my system. It would crash every few levels, and was usually preceeded by some strange texture artefacting:
Knowing my luck, of course, it's probably just my graphics card.