Mark Driscoll began to identify himself years ago as subscribing to a "limited unlimited atonement" or an unlimited limited atonement. The atonement purchased the entirety of humanity or creation away from Satan, sin and death on the one hand, but only had saving power to truly and eternally save the elect on the other.
Summarily, the unresolvable tension Reformed folks have spotted in this formulation is that the limited and unlimited elements are at odds. Why should the atoning death of Christ be unlimited in scope that it positively purchases humanity as a possession of God through Christ (which is redundant anyway since all creation belongs to the Lord?) if the EFFECTIVENESS of the atonement only applies to the elect, who are a subset of the whole human race? After all, since Amyraldians aren't universalists in practice all the non-elect will be consigned to Hell and that could imply a super-majority of the human race that has not heard the Gospel, couldn't it?
The pragmatic reason that may have lain at the heart of Mark Driscoll's advocacy of Amyraldian atonement was that he may not have wished to completely alienate those members and attenders who did not subscribe to the L in the proverbial TULIP. Driscoll also never took a positive stand as to what he was for in interpreting apocalyptic literature and eschatology. He'd make fun of postmillennial theonomistic types as wanting to rule the world without even being married yet on the one hand, and on the other he'd make fun of futurist/dispensationalists with charts as to why Jesus would be coming back next week. But that's shooting fish in a barrel rather than attempting to articulate what a person positively does believe and punting by saying that we shouldn't be too confident talking about things that haven't happened yet could be construed as defaulting to a futurist interpretation of Revelation without conceding the point, and this despite the fact that the historicist, preterist and idealist schools have not been addressed in any way.
If Driscoll relaunches his ministry later this year he might do well to say what he's actually for on this set of topics. If he retains his unlimited limited atonement schematic he'll have established that he's never ultimately really been "Reformed". He might be sympathetic to elements of Reformed dogmatics and systematics, possibly, but he won't be Reformed as such.