As a mapper, I can give you a few tips:
1: Start with an orange map basic layout. Use brushes to signify where the models are to be. Once you have the basic layout, you can start deciding what will you turn into a building, what will be a rock or a cliff and so on. You seem to be (like almost everyone else who's starting to map) spending too much time on making little details and making sure they look good (i.e. the building; but everything around it is just... inadequate. The displacements are rough, the skybox is connected to the playable map (but still, it's a beta so that's for starts acceptable. Otherwise giving player the ability to touch the skybox while on the ground = no no.)
2: Interlopers.
http://forum.interlopers.net/ If you're not already a member, become. This is pretty much the best HL2 mapping forum around. This, or maybe the tf2maps community, but I've never been there (it's tf2 specific so you should pick the latter)
3: Work. Work, work, work. Displacements for example are one of those things that seem hardest to do and the only way to be good is to get some experience with it. Open the decompiled gravelpit, pick a cliff and see if you can duplicate it from scratch.
4: The optimization guide.
http://optimization.interlopers.net/ This is your new bible. Read and abide by it.
Oh and another tip: Don't be too afraid of getting mapper's block; it only lasts a few months.
Or years in my case.
EDIT: Oh and at least mentioning if this is a 5 point cp map or gravelpit style or something wouldn't hurt.