Yes. DON'T USE AI IN MAPS. That is all.

+1. Also try to not use hightmaps for modeling. Let me find the post I made a while back..... AHA!
The two things you NEED to learn are: vertex modeling and plane modeling. Vertex modeling is usually used instead of hightmaps on the levels themselves. It almost always looks better and it always conserves polygons more. The only tutorials I know of are: http://www.youtube.com/watch?v=l3J3Dqt_cpA , and http://hce.halomaps.org/index.cfm?fid=3591. For plane modeling, it is usually used on architecture. It tends to be better looking and saves polygons also. A very good tutorial for that is found here: http://hce.halomaps.org/index.cfm?fid=3628.