This thread hasn't been updated in a long while, but the game development is progressing nicely so I thought I'd update it.
Development continues on the Door Kickers beta with another small update to fix a few things. The single mission system is already pretty complete, with just some small tweaks expected for the gear and tactics. They already have 72 missions and are expecting to add another 40-50 more. Also working pretty well right now is the mission generator. It uses the existing maps but adds random number of bad guys (although you can specify small, medium or large amounts) as well as the mission type (kill 'em all, hostage rescue, VIP escort, warrant execution), map size, and whether you have intel on the interior of any buildings. The gold version will have this mission generator plus one that instead of using existing maps will auto-generate one instead, so it will be truly unknown to a player.
The campaign has yet to be released, but apparently it's going to be a sequence of missions. During them your officers gain experience which raises their individual skills. You also get points to unlock new or better skills on a skills tree, also you gain points to unlock better and more exotic weapons.
On their Facebook page they teased one of their new maps, it's a 64 meter long ship:
click picture to enlarge
There is also an in-game editor which allows you to build missions pretty much like the official ones. Right now the only thing lacking is an easy way to build the background, which is the floor or ground surface. There is a decent default surface but you can get fancy and use some image editing to get much more detail if you wish.
What makes the game so much fun right now is the Steam Workshop support. There are dozens of user made mods, mostly levels and weapons. Red Panda has made some excellent ones, including one of my favorites, Extra Gear, which adds new items like glow sticks for your guys to carry. His levels are pretty good as well, I've play through several of them and they have some clever touches. There are other folks posting mods as well and the game supports adding, enabling and running mods pretty easily.
I'm not a shill for the developers. I'm just figured that this is one of those gems of a game that gets buried with all the other games in Early Access over on Steam, so I figured I'd be a bit of a game angel for them and talk the game up a bit. Even if you want to wait till release (which is now supposedly planned for October) it's still worth putting on your Steam Wishlist if this is a genre in which you have an interest.
UPDATE: I was going to mention this in the writeup above but it slipped my mind. As I have been playing Door Kickers I've been trying to think of a game to compare it to. People have mentioned the Sierra SWAT series and there is a lot of common ground there (of course, DK is 2D and top down, but it has a lot of the same feel to it). However, it occurred to me that what it really reminds me of is the early Rainbow Six games, again, just that it's 2D and top down. If you remember spending hours playing that first (I think it was the first) mission in the original Rainbow Six, where you laid out your plots for your teams to enter and search that museum and free those hostages, this has the same sort of feel to it. The maps aren't as large of course, but it does have a lot of the same characteristics of planning and execution that R6 did.