Just like the title says, my quest log is sitting at 23/25 quests yet every time I try to pick up a new quest it claims that my log is full. I even counted all of the quests and they do indeed equal 23.
We believe (though it hasn’t been confirmed) that they piggy back on the quest log for your daily emissary quests. Not the individual WQs, but the hidden quest to do 4 for a particular faction. You can see when you complete one, it magically shows up in your log until you turn it in.
I expect their attitude is that by the time you get to 120 and emissaries start being active, you shouldn’t have that many other quests hanging around.