Not sure whether this would result in an unreasonable amount of polling, but it would be nice at times to see the resolution of an image/icon in the statusbar next to the size or format, e.g. "my-pic.png (6.0kB) PNG image (1280x800)"
Has been removed in 1.5.3 because it caused (compared to other thing) massif amounts of io. http://git.xfce.org/xfce/thunar/commit/?id=34fdbb4a4b3ee4b3274389b58cd207d45f8349cc
Ah, that's why I had the feeling it was "missing" :)
Problem is that the complete image is read, not only the header. So you can image that's not really effective.
*imagine that it's not really efficient