I bet that fans who discovered MM or LOG were quite "surprised" in comparison to what they had heard before. I hear more musical "evolution" between communique and BIA for example, than between TRD and Tracker, but again it's just my opinion, and my hearing impression. Of course, I don't pretend to say the truth 
Same for me.
MM and LOG have a clearly defined identity, MM is the "rock" album, LOG is the "keyboard" album.
But from BIA onward I have difficulties to put a conceptual word to define them with the exception of both duet album and the NHB one.
agree. I would call LOG the "prog" album, compared to the "rock" MM
I think that concepts in solos albums are more defined by lyrics themes, rather by music.
for example, Guy said that many songs from KTGC take place during the 60ies, Mark said that many songs from tracker are about time in life, STP is mainly focused on America (not all songs though: WII, silvertown...), golden heart has many songs about kitty, etc... but I agree that it is less obvious than MM and LOG which have a "sound unity" so to say
