The "Compare Fork" option could be more useful.
Part 1: Diffs
I think a better default behavior would be to show the same type of diff that is shown in Pull Requests (See Issue 590), but if there are incoming changes, have a warning banner at the top of the page that says, "WARNING: There are also incoming changes from the parent that this diff does not take into account -- click here to see a diff that takes the incoming changes into account"
Part 2: What to Compare
By default the "Compare Fork" button seems to compare the head of default on both the fork and parent. This is fine, but there should be some way to change the endpoints you are comparing, similar to the Pull Request view.
Part 3: Creating a Pull Request from Comparison
There doesn't seem to be any way to be comparing a fork and say (directly from the comparison page), "Create a Pull Request from this Comparison". This seems like a logical thing to want to do (many here have missed having this option).