I must say, you've nailed the atmosphere and terrain very well. However, I'm not too sure about the gameplay.
I'll tell you what I meant by "spice". A game must make it so that the player seem to be really in control of what happens in the game, and that the player is the ultimate decision in the outcome of the story. This makes the players feel like they have a much bigger role.
So far, from my very limited play, it really doesn't feel like that it does. It is okay that they're the normal Marines with a few better ups, but they don't seem to fill the bigger role needed. The only thing that we could do is stim and attack in a group. In short, there's nothing interesting about the fighting system and nothing that drives me to continue playing other than to hear the rest of the story... which I felt was not enough.
The thing that you had going were the air strikes and bombing things. This is a GREAT thing to include, however, it is not controlled very well. For instance, in the beginning, it just randomly said "AIR SUPPORT wh00t!" and I saw some valkyries fly through, but I never saw where it actually bombed... that's no good :\...
I think it'd be best if air support can be called and directed by the player. This way, the players'd feel like they're more in control...
You can also use things like Grenades and all that.. to spice up the fighting parts...
I don't know... I haven't played much yet.. but yeah..
Btw, great story and atmosphere so far, I like it

None.