There's not enough information in the original maps to truly exploit all the features of a raytracing and global illumination engine. None of the textures have normal maps for example, so all of those beautiful bricks and such look flat as a board. Lighting was only defined ambiently at the entire sector level (think shapes on the map) so there's no information on light source locations, direction, spread, etc. So you're really just left with some fancy dynamic lighting and shadows--which IMHO still looks pretty cool (love the shadows and green glow as a BFG ball whizzes by) but yeah it doesn't look anything like a game designed with these features in mind from the start.