I finally got around to playing the demo of this. I have to say it was great fun and a very refreshing change of pace, but I did have some issue with it.
1) It took me awhile to realize there were switches on some walls that I had to use to proceed. This probably came from the fact that I associated the gameplay more with Descent than Doom.

(For this reason I would say a tutorial level would be very helpful -- or perhaps just a hint system built into the first few levels?)
2) I noticed you can "fly up" by holding the jump key (spacebar for me), but it's very slow, and there's no way to "fly down." I checked the controls menu but there are no mod-specific controls and the standard ones for swim/fly down don't work. This might not be something you can't fix, but it detracts heavily from the game, IMO. If the game is going to depend on you being able to manuever well in the air, being able to move up and down quickly is a necessity. It would be a shame if this couldn't be fixed.
3) There was a sizable learning curve involved in getting the shots from the ship to aim correctly and hit the (generally smallish) turret and flying enemies. Not sure how you can address this one, but of course any game focusing on flying players and enemies is going to involve more difficult aiming.
4) Some of the geometry in the first demo level was tricky to manuever around, and I got stuck often. In particular, the pits to the right and left of the starting area (each containing a powerup) were difficult to move in, and I got caught up on some of the pillars on the pyramid later in the level. Again, this could be a learning curve thing, but it's frustrating. Perhaps you could add invisible blocking lines to smooth out these areas?