Multiple projects open simultaneously
Premiere Pro use to not be able to open multiple projects simultaneously, but now that it does it has helped make my personal workflow so much better because I can easily copy elements from one project to another without the need to go to import an entirely separate project.
I think it would be just as helpful to be able to open multiple projects in After Effects the same way.
Adam Pippin commented
You can do this with After Effects by adding -m to the end of the shortcut target for After Effects. To open an additional After Effects instance, shift-click the icon.