Looking back on Mindcrime now, I think it holds up pretty well, hell it complains about the 1% ruling America, a problem that has only gotten worse in the years since.
Musically I don't think it was anything revolutionary, what made it stand out is that it was a complete concept album well told. Seventh Son had a few songs that dealt with a concept but it did not commit fully to the concept album cause like this one did. King Diamond was the master of concept albums during this time period but the music was more aggressive, King's singing can be an acquired taste and not everyone likes horror movies. Mindcrime musically is much more palatable to mainstream tastes.
I do think Queensryche was a one album wonder though, Rage For Order had its moments but I don't think they have done anything that even compares to Mindcrime in their entire history. I hated Empire upon release and only went to the concert for it since they were doing Mindcrime in its entirety for the first time since they were only an opening group when Mindcrime came out. Mindcrime II was a pale imitation of the original, although I give them credit for staging an interesting tour with the double Mindcrimes back in '06 when it came out.
Also thanks for making me feel old, I bought almost all the albums you reference in your opening bit in 1988. And for the record, other commenters are right, Conspiracy > Them and I would argue now is > Abigail, although Abigail deserves mega credit for reintroducing concept albums to the late '80s.
7-1 King Diamond/Slayer