There have been too many posts for me to reply to individually, but I'll go over the basic points I've seen crop up a bunch of times. I'll try to address everything as transparently as I can, because there's a lot of misinformation floating around.
Is this a leak? Was it released intentionally?
We had always been planning to release gameplay video(s) as part of the social media campaign, and we'd always hinted as such. Unfortunately, we ran into some difficulties while we were wrapping up the build (including several severe FPS drops and crashes) that delayed our plans for the media campaign and any gameplay videos. Once we got those sorted, we started to try to figure out what we could show off; the video that was posted to YouTube was a rough first cut outlining some possible action.
Someone showed someone else something they shouldn't have, and that someone else decided to share that something with the internet. And here we are today. Bottom line is that sooner or later, you were going to see what you saw; that's why we haven't tried to cover everything up or get it taken down. There's no conspiracy, we don't have any ulterior motives.
How old is the build of the game?
The build featured in the video is between four and five weeks old. The art assets, level design, lighting, sound effects and music, voice acting, weapons, gameplay mechanics, etc. are all basically final. You won't be seeing very many changes between what's in the video and what's in the final product.
Is the music actually in-game?
The music in the second half in the video is the actual song from the level, it hasn't been overlaid afterwards. It syncs up well with the action because it's been designed to; the song was specifically written for this area. The audio quality of the YouTube video doesn't do it justice though, and it sounds like the game's music volume was turned up too high while the video was recorded.
Why are there iron-sights in the game?
You can look down the sights of the .357 for a slight zoom and increase in accuracy at the expense of your rate of fire. It was a mechanic that was present in Half-Life: Deathmatch, but we've brought it into the singleplayer to help solidify the weapon's role. We've added an option to the main menu to completely disable that functionality, as a result of the feedback we've received from the video.
The .357 and crossbow are the only weapons that you can use the sights on, and they're for decent gameplay reasons. We aren't casualising or consolifying the mod; you don't have regenerating health, there aren't chest-high walls, you won't get strawberry jam on the screen when you're hurt, etc.
Why does everything look like crap?
The compression YouTube applies has blurred and darkened everything, even at 720p settings. You lose a lot of stuff like the scanlines on the HUD, the stars twinkling in the sky, the moonlight illuminating the launch pad and the hologram illuminating the control bunker, any real texture detail, outdoor characters, etc. I've got the original video at 1080p, and everything looks fine.
Why is the voice acting so bad?
Everything is intentional; the soldiers are meant to sound like over-the-top movie bad guys. We're not interested in trying to get you to empathise with the enemies.
Will this affect the plans for further media or the release?
It has, we're actually going to be moving some things forward. Time has never been on our side.