Wow, I read the whole thread and I still can't wrap my head around this. I have a ton of thoughts that I'm really trying to process...
a) MP's rationale for leaving the band makes a ton of sense, for a normal person. Does it really make sense for Portnoy, though? We know that he loves music of all styles, even if he's not as enamored with the DT brand of prog metal anymore. Combine that with his obsessive tendencies, and I still can't really believe this is it for MP and DT. I would put pretty good money on MP rejoining the band within, say, 5 years. Whether that's in a tour or album capacity (or obviously both!), I dunno.
b) In the interim, at least I really like A7X. I hope he continues with the band in a permanent capacity now that he's ex-DT. With MP, I think we can safely expect the band to be more metal and less...experimental pop rock? I think that if MP gets them to return to either their WtF or CoE form, that's a very good thing.
c) Who replaces MP? Obviously it has to be someone with major chops, isn't locked into another band obligation, and is capable of learning all the music - or at least enough for a set list. I think the days of the rotating tour set list are done now; how much can the new guy learn? More importantly...
d) Who's the leader of DT now? Is Petrucci or LaBrie ready to carry the band in a business, press, and fan relation sense? What happens to something like Prog Nation - is it over forever now?
e) How does MP leaving change the direction of their future output?
I think some of these things have been posted already, and I figure the rest of you are just as
as I am.