I find it quite annoying to assume I have a "window" seat, only to find that there is no window in that row. I think you're right about row 36 not having one. Also, several rows ahead of 36 are missing windows. The 747 must have some wiring or duct work in these locations that prevents placing windows there.

Someone else correctly noted that the 742 has a "cutout" in the bulkhead on row 21, so you can store small items in front of you similar storing under a seat ahead of you. The 744 does not have this.

Still, I find row 21 desirable on 742 and 744 alike, and no problem with legroom there. In fact, I can usually get up from the window seat and get out without disturbing the other passengers in the center and aisle seats. The one thing that would be great in row 21 however, would be a little grab handle on the bulkhead, near the aisle, so that when the window passenger gets up, he can grab hold as he is stepping over the other passengers' feet on his way to the aisle.