I was trying to watch a defect I had passed on to someone else and noticed that I can no longer do so from the full view. Was this removed intentionally? I see it in the Quick View mode, but doing that every time I want to watch a defect/user story is seemingly unhelpful.
I can still see the watch icon on the Defect Full View so perhaps there is something amiss with your User rights? Check to see if you have Email Notifications enabled for your user? Maybe CA made the button intuitive based on that setting on the full view page but not on the quick detail page?
We currently are tracking down a Defect related to this issue. I will create a Support Case for you so we can notify you directly when it has been fixed.
It happens when users are using the Personalized Navigation. You may still add a Watch to an item via the gear icon on the left side of the row when using the Iteration Status view or when using QDP.