Well you could look at it from this perspective...
Making new textures is hard work and takes a long time and you can not guarantee your textures will be anywhere near as good as those made by the professional GM team!
For a map in my opinion you need an assortment of:
choke points - but you got to be careful these are not too hard to get through
places for cover
sniping points
enough routes to an objective but not too many so that it's easy for an attacker to slip past you
realism visually and technically - make it look like you could be somewhere real. Yeah sounds obvious but little things like - why is there a corridor going down here? If this was a real Axis installation, what purpose would the building and it's rooms have? I think some people are guilty of just laying a map out in a way they think would look cool and play well, but if it makes no sense from a "is this a realistic layout?" point of view, then people will notice - maybe not consciously but in the back of their mind there'll be a niggling doubt!!
My final point I feel is that you should try and make every part of your map look a little different. If you have corridors, try and ensure that each corridor has a different visual slant to it or different layout. Again, sounds obvious maybe but you often run down corridors in custom maps and think "hey this looks like that last corridor I ran down!" followed by "hmmm this map is a bit bland!"
Oh yeah, and make sure you vis your map well or you'll get bad fps by people playing it and your hard work will have been wasted. If you don't know about vis take a stroll around some forums and search for "vis", "portals", "caulk", "hint", "structural brush", "detai brush" . And do not add another thing to your map until you understand these concepts!! Seriously, you'll appreciate this later. Check out
www.nibsworld.com, he has a nice tutorial on vis and hint brushes etc.