I think what makes something progressive is challenging the norm. Pretty much doing the stuff that you feel like can be ambitious that no one really had any idea that you were going to do it at all. Back to say like, last year, when we were all talking about what sort of sound that DT13 could go into, some were probably concern that DT would not do anything ambitious for a fear that DT would not have its signature DT sound and play it safe to that. Fast-forward, they did something like The Astonishing where, yes, some people probably thought it would be a full-on concept album, but I do not think they ever would imagine that the whole concept would be this elaborate.
Another note, to me, progressive music can be classified as putting everything on the line, maybe risking fanbase alienation, maybe some die-hards probably would not like it, maybe even hate it, but the band did it because they felt that this was the direction that they want to explore and enjoy and they are putting all on the line (fanbase, finance, reputation, etc.) to achieve that. I think Dream Theater should be proud of what they had constructed in The Astonishing and if long-time fans do not get it, oh well.
Just my two cents.