Here's a poor analogy that I thought up of earlier today when thinking about this topic - Portnoy is more of the Arts, while Mangini is more of the Sciences. What I mean by that is that MP's drumming is done for show, it's flashier and more entertaining, there for audio and visual amusement and fun, creating a sound that is engaging and exciting to everyone. MM's drumming is methodical, scientific and well-thought out, very formulaic and strategic, laid out in patterns and mathematically pieced together to sound consistent with the rest of the band's rhythms.
Both have their pros and cons, but I think what we should take away from this whole "debate" is that, those of us who were tired of MP's drumming were more likely to find MM more appealing, whereas those of us who find MM's drumming to be out of reach for understanding were probably fans of MP's more showy and bombastic style of drumming. This assumption is probably way off base, but I figured I'd put it out there to see if anyone agrees.
As a drummer of over 20 years, I've grown to appreciate very complex and technical drummers, people like Thomas Lang and Gavin Harrison, but Mike Mangini takes the technicalities to new levels, and his explanations only solidify that feeling that he's very scientific in his drum-part-building, whereas Portnoy is a "bag of tricks and tools" kind of drummer, who will do what the song needs but won't be afraid to color around the lines, so to speak, and I think that's what a lot of fans like about his drumming.
-Marc.