It may just be me, but I much prefer the big rooms with obstacles and lots of baddies - they let me exercise a lot more strategy. The narrow corridor rooms just make me think "oh crap oh crap oh crap I'm going to get hit and there's nothing I can do about it!"
Perhaps in the sequel (if you do one) you can add moving platforms and rooms that change structure. Conveyer belts. Maybe a garbage compactor like in Star Wars. Muahaha.
edit: Also - perhaps more visual cue from the grenades? Maybe a little circle with trails. Also, try getting a few more explosions and picking them randomly instead of using the same one each time.