A factory restore will delete anything that you have saved on your Glo. If you only buy books from Kobo, then it shouldn't matter because everything will be restored from Kobo's servers. If you added your own books (using Adobe Digital Editions, Calibre, or you copied them over yourself), make sure you have a backup copy of those books on your computer. If you use Adobe Digital Editions, for purchased books or library books, you may want to backup the activation file too. (Adobe limits the number of devices you can activate, and a factory reset makes it think it's a new device unless you backup and restore the activation file.)

I have heard of troubles downloading updates during the registration process, but people around these parts will help you out if there are problems.

Given those caveats, it's perfectly safe to do a factory reset.