The answer to that is yes and no.
Maybe I can shed some light on this, having played the original beta and also when it was live on GameTap.
It depends on what you mean by MMO. No, it's not WoW. It's more of a graphical chat-room. All of "Ages beyond Myst" is here. (Ages beyond Myst is the original beta live game, altered for offline play and with a couple of ages that Cyan finished before canceling the beta.) Almost certainly there will be some added content from when the game was available under GameTap. (At least most of the files are included in the download).
When the game was actually live, there were monthly events put on by Cyan staff. These included plot and storyline events, opening of new areas, multi-user puzzles to unlock new content and from time to time, new clothing for avatars. These almost certainly will not happen in the foreseeable future.
The big draw, however, were user created events. There were regular meet-and-greets, poetry nights, dance parties and heek competitions (Similar to rock-paper-scissors but with up to 5 players.) There were D'ni language classes, tours of Bahro glyphs and paintings and discussion groups, especially about in game history and working translations of D'ni scripts and books.
Cyan released several new ages during the course of their run on GameTap. Some of the puzzles were altered to make them only solvable in groups or with at least two people. There were also at least ten other ages in various stages of development as well as quite a few fan created ages that are still being worked on.
So, if you are looking for fighting, leveling, auction houses and monsters, this probably isn't the game for you. If you like adventure games, long discussions, thought puzzles and linguistics, this might be something you would like to try. Don't expect any official updates, new content or bug fixes in the foreseeable future. Supposedly, the source code will be released. There are quite a few very capable coders and programmers in the Uru community so there is the possibility of fan created patches and updates before to long.
It's free. If you don't like it, that's fine. If you do, maybe I'll see you at the next poetry slam or Guild of Archivists meeting. Also, think about dropping a donation on Cyan. The more money and support they get, the more likely we will see real progress.