Gnus was designed to be fully compatible with GNUS. Almost all key bindings have been kept. More key bindings have been added, of course, but only in one or two obscure cases have old bindings been changed.
Our motto is:
In a cloud bones of steel.
All commands have kept their names. Some internal functions have changed their names.
gnus-uu package has changed drastically. See Decoding Articles.
One major compatibility question is the presence of several summary buffers. All variables relevant while reading a group are buffer-local to the summary buffer they belong in. Although many important variables have their values copied into their global counterparts whenever a command is executed in the summary buffer, this change might lead to incorrect values being used unless you are careful.
All code that relies on knowledge of GNUS internals will probably
fail. To take two examples: Sorting
changing it in any way, as a matter of fact) is strictly verboten. Gnus
maintains a hash table that points to the entries in this alist (which
speeds up many functions), and changing the alist directly will lead to
Old hilit19 code does not work at all. In fact, you should probably
remove all hilit code from all Gnus hooks
Gnus provides various integrated functions for highlighting. These are
faster and more accurate. To make life easier for everybody, Gnus will
by default remove all hilit calls from all hilit hooks. Uncleanliness!
expire-kill will no longer work. As a matter of
fact, you should probably remove all old GNUS packages (and other
code) when you start using Gnus. More likely than not, Gnus already
does what you have written code to make GNUS do. (Snicker.)
Even though old methods of doing things are still supported, only the new methods are documented in this manual. If you detect a new method of doing something while reading this manual, that does not mean you have to stop doing it the old way.
Gnus understands all GNUS startup files.
Overall, a casual user who hasn’t written much code that depends on GNUS internals should suffer no problems. If problems occur, please let me know by issuing that magic command M-x gnus-bug.