What stops me is primarily the fact that I'm not sure how I'd go about it, although I admit there's a nagging feeling of absurdity in there as well. On the other hand, nagging feelings of absurdity have never stopped me in the past, so we can mostly discard that as a problem.
The prime bird spot is about four feet outside of my sliding glass door. The tripod is just inside the door, with camera on it. When a little bird comes to the feeder, however, they can see me as easily as I can see them, so I am forced to do a commando-style skulk to the camera, trying to stay behind the vertical blinds, reach out and hit the power, focus, and by then the more nervous little birds are getting mighty suspicious of all that skulking going on and relocate somewhere safe. (The mockingbird and all the bluebirds don't care, mind you--they're confident they can kick my ass. But I'd like to get photos of some of the shy birds, too.)
Today I've got the camera hidden behind the vertical blinds, with just the lens protruding, but there's still some motion going on, since the blinds are not a solid wall, and the gap caused by the lens pushing one aside reveals the top half of a chunky nondescript woman trying to be sneaky, which is undoubtedly causing a great deal of amusement for the little birds. It does seem to work a little better--I've got several shots of tufted titmice, including one rather scruffy individual missing the tufted part. (I'm assuming molt.) However, it's still not all that great.
So how would I construct a bird blind? Ideally the sort that one could take down easily. All I can think of is taping opaque paper to the inside of the sliding glass, with a slot cut for the lens, and I don't know if the lens, which requires a certain range of motion, would require a hole large enough that my laughable attempts at stealth would still be obvious. Anybody have any thoughts?