I guess I don't use enough donation based services to know what is the "norm" - other than Paypal receipts I don't recall getting any acknowledgment from donations I have made around the place.
With my donator hat on, personally I'm fine with it, it helps preserve my (very much) illusion of anonymity and that whoever is at the other end is not secretly collating my paypal email or putting me on some distribution lists etc. However I can totally understand that to others it might appear that the donation was not personally appreciated without the response.
As Kovid has said, he makes his living from the donations so you can bet he appreciates each and every one for sure! However if you look at the sheer volume of support queries alone the poor guy answers across multiple mediums (forums, bug tracker, emails, patches, facebook, PMs, visitor messages, and probably others I don't know) it is quite remarkable he gets any development done at all...
I could post a picture I guess of the excited little "ooh, nice - thank you..." I exclaim any time a donation email comes in, or a video clip of my happy dance, but that would just scare small children.
