It would be nice to be able to set the favicon on simple web apps built through AGOL without having to hassle with code.
The new Open Data 2.0 sites do it quite nicely as it is a property of the site. (see image below)
This is by no means a show stopper but the less code I have to deal with the better. Seems like it was an simple non-functional requirement that will eventually be dealt with. (as of 2/15/2017).