Great base
I got a serious Diablo 2 vibe from this. Good graphics, very fluid, no problems with the aiming system... this is an excellent foundation.
As for what stands to be improved... a pause button is very important. Also, I'd like to see a roll/backstep command, something to move the player quickly for a second. Other than that, basic expansion like more weapons/maps/enemies could help. Great job, though!