There was a silly topic regarding this elsewhere recently. Progressive does not have exact boundaries. Time signatures is not a key element, in some progressive bands it is, in others it just the trip they take you on. For example Pink Floyd was a progressive band yet their time signatures were at least mostly 4/4. Im not going to run through their libary to double check. They also didnt go off into long syncopated flurries. Just one example but trying to explain all the various things that could make a band progressive is baffleing, its something I just hear and dont question. Some people want to pin it down to the exact Webster defination of the word, meaning its all new, never heard before, therefore representing progress. By these silly standards - a possible example - Kurt Cobain would have been progressive ? grunge ? I could name other examples but would get me in trouble and reopen old cans of worms.
For me I guess I hear it as how a band takes you on an adventure through their music regardless of the level or depth by which they explore the technical aspects of music. The music becomes a painting, it could be a hyper abstract painting or it could be a scerene landscape, or it could be a band totally out of the box, but thats rare and getting harder by the year, plus it would allow just plain off the wall stuff that exposed little talent or deep thought.
At one time this type of music went by other names, like "head music" "art rock", just a few I remember, there may have been more, but I think somewhere along the lines it was just settled on progressive to avoid another dozen sub genres. Unfortunatly with bands like Dream Theater applying intence musical knowledge, ability and virtuosity to their form of progressive metal many people think thoese elements are the only defination. But its not, its about the "trip", about the "painting".
Other early examples I can think of regarding the time signature issue, and excuse me if I wrong, I dont spend much time counting measures. My own stuff apparently, according to drummers, at times bounces between 6/4 & 4/4 so Im no counter. Jethro Tull - mostly if not all 4/4, same with Kansas and I even bet alot of ELPs stuff was 4/4. Rush mostly 4/4. Yet there was few similiarities in these bands.
Another thing thats causing confusion and discomfort with the term progressive these days is the fact that many different bands within their genres are applying various elements commonly found in progressive______ . So it can quickly become a giant mess if people want to go there.