I was thinking about just this the other day -- why and how is QAnon a different sort of conspiracy theory?
One thought that occurred to me is that it is not a post-hoc theory to explain events in the past, like for example, 9/11 trutherism, the Kennedy assassination, COINTELPRO abuses, or Roswell. Those are "classical" conspiracy theories, and they often touch on shady machinations of government and extra-governmental sources of power, but they focus on a major fact, event, or sequence of them in the past.
QAnon is predictive. It is not accurately predictive at all, but it is fundamentally present-and-future-oriented. People buying into it believe that they are in the midst of a vast conspiracy now, and expect things to transpire in the future based on that version of reality.
That could make adherents act differently today and tomorrow from how they would otherwise, expecting certain rewards later or to be regarded in a particular light "when the truth comes out", instead of merely feeling differently about actors from the past and believing they have uncovered some past truth.
Another aspect that I think plays a key role is the wink and nod support from those with power who know it helps maintain their grip on a certain segment of their voter base. The tacit support given by seriously empowered agents in our world is stunning and upsetting, but rarely if ever crosses a line to where they've really committed themselves.
Then, there's the crowd-driven nature of it. It's like someone harnessed the concept of "anonymous" from a few years ago and deftly weaponized it for a specific political purpose. Whatever is said by Q may not have been said by Q. It may have been said to throw someone off of the trail. Anyone could claim to be speaking as Q. No one knows where legitimacy flows from. It's a headless thing, but it can't stop talking -- who knows what it might say? Who might use it to speak?
It has clearly already radicalized a large number of people into rejecting many normal beliefs -- how far could it direct their actions at large, or at least those of individuals?
It is a seriously great con. But these aspects -- future-orientedness, tacit legitimization, and potentially zero control -- make it a seriously scary thing to have built into the political landscape of today's society.