Definitely the Mobile Automated Siege Cannons (MASCs). These are small tanks that the marine commander can construct, order and deploy to siege alien hives and bases.
What can you tell us about the game's level deformation feature?
We have "dynamic infestation", which is a bacterial goo that grows wherever the aliens go. It will break through welded doors, turn off lights, destroy computers and generally create chaos wherever it goes. Go to http://www.unknownworlds.com/ns2/videos/ to see it in action.
What other features do you think are important?
We have four "pillars" that we're designing the gameplay around (link). The first pillar is "Two Unique sides". Both sides (Marine and Alien), are totally unique. For instance, when playing as an alien, we will increase the field of view and blur the edges of the screen a little to make it feel like you have a different kind of animal perception than the humans.
The second pillar is "Real-time Strategy". This is where our resource model, tech tree, buildable structures and Commander mode comes in (top-down view used by some players).
The third is "Dynamic Environments" which basically means the game will never play the same way twice. The dynamic infestation is part of this, as are weldable and breakable doors (for dynamic map routes) as well as lights, power sources, lifts, etc. that respond to the game events.
Finally, "Unlimited Variations" is our modding architecture. Our entire game is written in "Lua" script and will be released with the game, along with our level editor, script debugger and everything you need to modify NS2 or make your own game. Adding a new weapon is less than 100 lines of script. You can change the resource model, add new AI units, change any UI element, etc. Our game engine has no concept of NS2 at all - everything NS2 is written in script. So everything from Commander Mode to player movement code to weapons is all effectively open source and shipping with the game, so you can be sure players will add all sorts of new game modes, abilities, etc. Maybe we'll even get our own "Counter-strike" on NS2!
You recently made the decision to create your own in-house engine instead of using the Source engine (you can download a videocast with the first footage from the engine at Big Download - editor). Why was this move made and can you tell us about the engine's main features?
This was to serve our dynamic environments and modability desires. We had difficulty getting dynamic infestation working in Source and we didn't need most of the features in it anyways. We kept finding ourselves ripping out code we didn't need and adding a lot of code we did and it just wasn't efficient.
The engine's main features are it's fully dynamic lighting, ease of editing (our editor looks like the game with no compiling or pre-processing) and efficiency even on low-mid end machines. To get an idea of visuals and performance, check out these two blog posts. (here and here - editor)
At the moment what is the status of the game's progress and what release date are you targeting?
We do have a release date in mind but it can only hurt us to talk about it, so I'm going to keep my mouth shut. :) The game has to be awesome before we release it.
Finally is there anything else you wish to say about Natural Selection 2?
I'd just like to thank our community for their support over the years. Because of their enthusiasm and support, we are "living the dream" and making the game we want to play and doing so on our own terms.