I've been a Kindle user for years, but have had some frustration with Amazon lately and am thinking I want to diversify a bit by buying some other eReader. If I understand correctly, both Nook and Kobo use Adobe Digital Editions. If that is true, does that mean I should be able to load books bought from Barnes and Noble on a Kobo (or books bought from Kobo on a Nook)? Kobo seems to allude to this, especially since, unless I'm missing something, the Nook store and Kobo store are the only two major Adobe Digital Editions options. On the other hand, I haven't found anyone one specifically speaking to Kobo/Nook compatibility.
Both NOOK and Kobo have a variety of book formats in common, such EPUB and Adobe PDF. Kobo can use Adobe Digital Editions, but Barnes and Noble states, "NOOK Books from Barnes & Noble cannot be read on your Kobo eReader." However, B&N states the converse works, i.e., transferring ebooks from Kobo to NOOK. It is possible to read DRM NOOK books on Kobo by the legally-questionable removal of DRM.
That said, for books without DRM, such as the many public-domain offerings on Project Gutenberg, Internet Archive, and publishers of copyright-protected, but not DRM locked, ebooks, such as Baen Books, texts can be transferred between devices using software such as the excellent Calibre application.
Using Calibre or other tool, it's also possible to change the format of a book, such as Kindle's MOBI to NOOK and Kobo EPUB, or vice-versa. (Note that PDF is somewhat a dead-end: while it is easy to convert documents to PDF, going from PDF to another format is not always successful.)