[Quick update] It looks like Dropbox isn't the only AppIndicator that doesn't work in Ubuntu 17.04 Zesty Zapus (under Unity) due to the change of XDG_CURRENT_DESKTOP from "Unity" to "Unity:Unity7".
Update (thanks Martin): the Dropbox AppIndicator no longer has this issue.
Electron applications (such as the new Skype For Linux, WMail, PB For Desktop and many others) are affected as well, but in a different way. For Electron applications, the indicator is not displayed at all in Ubuntu 17.04 Zesty Zapus under Unity.
The fix is similar to the one applied to the Dropbox indicator. Simply run the application with "env XDG_CURRENT_DESKTOP=Unity". For example, to start Skype For Linux, you would use:
env XDG_CURRENT_DESKTOP=Unity skypeforlinux
To make the fix permanent, copy the application desktop file from /usr/share/applications/ to ~/.local/share/applications/, then edit the file and change the "Exec" line by adding "env XDG_CURRENT_DESKTOP=Unity" (without the quotes) immediately after "Exec=".
Some applications are set to start automatically and in that case, you'll have to edit the desktop file from ~/.config/autostart/ in the same way.
Note that some applications overwrite any changes made to their autostart files, located in ~/.config/autostart/. A way around this is to rename the autostart file, then in the application settings, set the application not to start on login. This way, the modified autostart file will be used (which has a different name and contains the workaround).