Looks interesting so far. I like the first part especially, looks like some nice combat there, having the combine up on the roofs and all. Proportions and such seem to be fine there, too.
The last part - big building, excavation entrance - doesn't really do it for me though: the room is too large to make much sense and the antlion combat seems less interesting. I think it's the lack of layout there, and the fact that it looks like an endless stream of antlions while it isn't - most players will expect antlions to continue to pour in, as that often happened in HL2 and it's episodes.
Those 'rules' are something to keep in mind: while you can create your own rules and expected behaviour, going against an established rule can confuse players somewhat. If you get what I mean.
As for visual detail, I know you're not in that stage yet, but it might be a good idea to sketch out some styles for yourself. How is that area supposed to look, where is the player, what looks fit well with that theme? What scenery can one expect there?
However, I think it's good to continue tweaking the gameplay for now and adding some more area's, to ensure that you don't loose sight of the big picture. Keep it up!
