Reduce your D800 images back down to 12mp, and you'll see they will look the same as the D300 images, most likely better.

What sort of images are you trying to shoot?
If you're shooting landscapes or detailed images f8 and narrower, you'd want to be shooting on a tripod anyway. DOF should not be an issue at f/11, I'd be very curious to see a sample image.

If you're shooting portraits or sports, you'll generally be at 5.6 or wider, which then comes down to a matter of nailing focus for sharpness. Also crank up the ISO a bit so your shutter speed stays fast to avoid blur. ISO quality was the biggest jump I saw when going from a D300 to D700, you should feel free to indulge without loss of quality.

Bottom line, unless you are locking down on a tripod, using live view focus to nail a detailed shot, a 1:1 view of a hand held 36mp D800 image is most likely going to look less sharp than a similar 1:1 view of a D300 image.

Visit my Nikonians gallery.