TL;DR ISIS is a doomsday cult analogous to David Koresh's. They interpret the Koran exactly literally and any deviation from it is punishable by death (this includes AQ and all shia for modernization offenses). They believe they are a primary actor in the prophecy that'll bring about the ultimate battle with "Rome" at Dabiq, Syria and the judgement day.
My take is that while modern mainstream Christianity laughed off the Branch Davidians as crazy people who offered nothing, mainstream Sunnis see IS as similarly crazy with the important exception of "well at least they're fighting the Shia". In other words, there is some tacit support for IS based on millennia-old factionalism, (edit: which is how they've grown from a small cult into a significant movement).
An interesting point in the article is that while AQ can hide underground and never be fully eradicated, IS has to maintain land and provide government services under the caliphate to remain legitimate (according to the Koran, of course). This makes it an entirely different enemy that can, in theory, be destroyed.
Further, no peace can ever be negotiated because the borders of IS have to keep expanding or the caliphate is no longer valid. That is, Muhammad says all non-believers have to be converted, full stop.
Anyway, the article is outstanding. Take an hour to read the whole thing.