Nice map for your first outing! Some thoughts:
1. good use of textures overall
2. good idea to choose DM as your first Quake map to work out the process of creating... the Quake SP community is much more active than MP for the most part. I switched from making only MP maps to SP and it's better overall as many more ppl play my maps. Something to consider. func is really geared toward SP maps
3. your lighting is quit good for an old school style map. I encourage you to look at ericw's tools website to see what can be done now with lighting. (I am currently working on lighting tutorials.
4. I really liked the small touches in your map. The uneven walls and floors, the door mysteriously cracked open in the other area. The warped woods planks were cool and I am stealing that idea!!!
These all give your map life.
5. some of the areas felt a bit the same though so easy to get turned around. You can vary lighting and / or textures to give players signposts. The player should never be lost in a level.
6. congrats on a very nice looking level
7. keep mapping for Quake! you are off to a very good start.
bonus question: are there leaks? is this map vis'd? I only ask because I had a weird framerate issue here and there. It could be my computer tho.