I think some times people are very impatient. It can take 60 seconds to quit to desktop sometimes. I once stayed on an infinite load screen for 12 minutes and it did actually load. I do this so my add-on savedvariables don't get ducked up. Also, people: please use minion and make a backup every time you are done playing. And copy that backup to cloud storage like google drive.
This has something to do how long the client was running (maybe clearing up resource allocation?) as I noticed that if I login for only a short time (like doing trait research and horse training quickly across a few character) then afterwards the client quit without any problem. If have done any longer gaming session, then it just hangs on quit.