Too simple layout... randomly placed sceneries maybe? Well, this isn't bad, but need some improvements. Try to do a better layout (More polys) and select the sceneries that you want before place them (Don't put them randomly).
You can do your own sceneries if it is necessary...
Those clouds will make lag and make a poor gameplay, so a suggestion is that remove them and replace them for fog.bmp and put it in the ground. Iy you want clouts use the cloud2.bmp (The default one)