Here are a few tips.
Try to use a few more polys so you can smooth things out a bit. Long straight tunnels and a bunch of unconnected rooms can hunder DM play with easy sniper spots and no place to hide.
Those triangular polys are bad imo, again, round thouse out with a few more polys and it can also help when you go to shade.
Scenery usage: Make use of the front, middle, and back options. Play with the transparency and color to give the illusion of depth. Also, rotate the sceneries so they're not the same repatative placements over and over. Clustering crates toghter, for example, can create nifty places to hide and defend yourself.
Go for a circular flow, like in DM maps like Jungle. Too streached out or too much space can make things boring.