What reminded me about it was the way the mobile tanks could 'hover' much like the way you move in your game. They released a game on ps1 http://www.youtube.com/watch?v=Nl3jbZ7JJ8I if you used a similar sound effect when the tank is sliding i think it could add a bit more atmosphere. (example around 3:15 if you want to get straight to it).
I'll give you a quick run through of why i suggested the levers based on my how my experience/fun changed in realtime while playing.
1st minute: really enjoying myself, strafing, being paticular with my shots to not overheat - always moving to not get ganged up on.
2nd minute: The experience of just moving away and shooting is not holding me as much - this might be because the shields i find are extremely forgiving, you can pretty much charge your way (through enemies) and get out of most situations fine (Then wait a sec for full shield again).
Its at this point im looking for another objective to hold me. The act of finding levers (or maybe a key instead?) is something that players should always run into by chance because to avoid death they are moving all the time anyway.
Then the strategy of 'choosing' when to descend becomes more important.
The way i see it, employing a strategy like this provides the major benefits of;
1. Allowing the player to control the rate of difficultly directly, instead of having to pass easy stages to get to the desired skill-to- difficulty-ratio. (I could probably stay on that first stage for around 10 mins - but i wouldnt be having fun until it was incredibly overwhelming.)
2. Giving players another strategy to play, build yourself slowly over time - or my personal fav, charge GUN-HO! ;)
3. Making the player feel like they are achieving more than they actually are. (reffering to picking up the lever/key by chance) - You can give the players the illusion of extra objectives. (this doesnt have to be explained in the tutorial either if when they pick up the key for the first time it says "now find the door").
(I should point out that im treating your game as I would any game that I played on kongregate - If i was determined to play your game for over an hour, I might discover new levels of enjoyment - but im really focusing on holding the player to get them to that point.)
Wow this is getting long, but just as a PS: Does the chrome bug affect movement when a key is press instead of the mouse? for instance if you used space bar for shooting?, its a comfortable enough position for the thumb and should be fine for long term play as long as it doesnt get to the point where you are rapidly tapping, but the overheating kind of prevents this.
Again, awesome game.