I haven't really looked at the Tomb of Horrors for 4e yet to be honest. From what I can see, personally the roads, paths and rooms seem overly angular to me. Another thing is the resolution of some of the textures used are blurry or stretched in comparison to others. Also try to find endless texture or make your own, this would prevent the evident lines formed from the edge of the source texture used for the cliff or escarpments. Very minor things to be sure, aside from that is just adding polish to what you have. Then again I'm pretty inexperienced myself and could be wrong.