what i learned from my tractor adventure is, that you shouldn't make the vehicle too overly detailled. the original tf2 vehicles are rather simplistic and only hint certain details.
i don't know how you're doing your lowpoly model, but if that turnaround of the
subd-cage is your actual lowpoly model, then you should change quite some stuff around, since there's a lot of supporting edges that you don't need on the actual low poly model.
the tf2 models themselves are relying on bevels here and there to make the actual edges not as sharp as your usual lowpoly models - which means you should go over all your actual edges and adapt them a bit to make them less visible. also make the round things really round. thats one of the things why the tf2 stuff looks so viable. they model the round stuff really round
your toplights and tires especially need a lot more roundage love.
but like i said, i don't know if that's your wip lowpoly or just to show the sub-d base.
and as i posted earlier, i think you should really hop into the model&textures forum located here and create a thread:
http://forums.tf2maps.net/forumdisplay.php?f=45
because this wip-thread is really for a wip from time to time and not hourly upgrade to your models
it's more on the map-side of things and even there not as often as you did with your vehicle
also you'll get much more attention and praise and fame in the model forums