Its a good layout, but as you say, very under-detailed.
What to do - invent a story for your map, where is it? a granary? a farm? an industrial building?
once you know where it is add elements (both brushbased and models, tf2 models could go just about anywhere in the real world so thats handy) that reflect that, make it dead clear to the player where they are.
You have some dissalignments in your brushes where the ramp meets the very top balcony bit.
You should kill the player once they get to the botom of the bit (on impact) rather than half way down. (it wouldnt be so bad if it weren't for the massive bone crack whilst in mid-air)
You should add at least one other resupply for each team.
Perhaps give two bridges across the pit, at right angles on different levels?
Give a definat visual reason for the bridge to be a certain shape, if it is flat give it supports so that you can believe it would work in real life, if it is curved up give it appropirate supports that go with curved bridges...etc
Make your lights less evenly spaced, add a few windows here and there, light fittings, areas of darkness (not pitch black though)
make the inteligence area look like it is an inteligence room. Add destinct markings to clearly show the n00b players on your map where everything is, like the black and yellow striped lines around the intel or the big blue/red arrow signs on either side of the bridge.
Get to it