Reading back from the beginning of this thread I realize that there are as many different outlooks on equipment upgrades as there are people using it. One important item still missing is what the XSi replacement will be used for. Also, which features seem particularly outdated or are simply missing? The XSi is still a quite capable body and of the older Rebels is the one that I see other folks using the most often.
AF and weathersealing were mentioned earlier as reasons to upgrade which would be enough reason for many to upgrade. Were there any other items on the wishlist?