I don't have Halo in front of me, but I seem to remember a lot more
detail. Even if there wasn't, I think you could afford to make
the map a little hilly... the map would probably benefit from the bases
being placed higher than the river in the center.
I like where you were going with those throwable rocks. A few
more prop_physboxes would go a long way in showing of HL2DM's fun
physics gameplay.
The only serious problem I see is players camping those base
roofs. You placed a nice amount of health and armor, and that EMP
Rifle with 3 energy balls. That's just overkill, someone could
knock away those dumpsters and have a nice 20-kill spree before anyone
could do anything whatsoever.
Last, I think that this map could benefit from this tutorial on improved teamplay:
http://www.snarkpit.com/editing.php?page=tutorials&game=HL2&id=149
It has a few problems, so if the author doesn't update it by the time
you look at it, please PM me and I'll send you a VMF that uses his very
creative technique properly (allowing for normal HL2DM, not just
teamplay).