My guess is that most of the folks that will be disappointed are those that think 5E will, or at least should, resemble a better version of their edition of choice and/or are overly attached to concrete/literal elements of their edition of choice.
When Mike & Monte say that you will be able to play in the style of different editions with 5E, they mean just that: in the style of; not a re-creation of an older edition, probably no "Save vs. Petricification, Polymorph and Putridly Pestilential Polyps." In the style of means the spirit of the law, not the letter of the law.
Some may also be disappointed that are overly attached to a specific aspect of a prior edition being part of 5E core. For instance, it is possible that 4E-style Defenses are part of 5E core, but they could also be chucked for Ability scores. I like Fortitude, Reflex, and Will, but why not just use Con, Dex, and Wis? My sense is that, with 5E's core, they're trying to dial it back as much as possible that there will be more freedom in terms of play style and modularity.
Finally, there might be some variation of Obamadisappointment. At this point, 5E sounds too good to be true; Mike & Monte are talking a good game but might be shooting higher than is possible to reach. This, of course, remains to be seen and I certainly am hopeful that 5E can be what they say it will be!