There's two parts to this.
Firstly, the screen is split down the middle, with each side showing a slightly different image for each eye. This is how you get the 3d effect. It means that you are only seeing the image at 1080x1200 resolution, although you get two slightly different perspectives of the same image. Special lenses in the headset make each half of the image "fill" up your eyesight and make it seem like you're really there (at least up to ~100° FOV).
Secondly, on that smaller half resolution screen, you are seeing a wider field of view than you probably showed before. The horizontal FOV rendered in the headset will probably be somewhere between 100°-110°, whereas most FPS games are somewhere between 75°-90°. That means that the number of pixels per degree is even worse.
So you've got a wider FOV crammed onto a much smaller resolution screen. I found the DK2 to be *incredibly* immersive and fun, but very difficult to read gauges, see runways in the distance, or read HUD information. I'm hoping the CV1 will be an improvement but what I've read says that the resolution isn't a massive jump, even though the screen-door effect is greatly improved.