So my suggestions:

-I know it's still a WIP, but laying out some scenery would be nice.
-Falling Damage needs taken out or something, because falling off small ledges and crap is really annoying.
-Bland empty pit as said before.
-The builder sentinels or whatever, need to go, and so does High Charity. They really pull from the beautiful skybox. Not to mention that there is no lens flare where the sun should be, yet you still have all the little flairs that appear when there is a sun.
-Waterfall... although it's really a good idea, It doesn't look very good... Is it a bunch of separate planes with the waterfall texture on it? If I were you I would just make it one plane with the waterfall UV'd in a pattern. Would look better.
-You can shoot the overshield and it falls right through the little platform under it.
-Also you can pick up the fuel-rod cannon through the platform.
-Blue laser bolt thing that shoots up has a flare, the red one does not. Looks silly.

That's all for now. It's really coming along though.