I'm not sure that mblase has it right in saying that the phrase "Kingdom Come" comes from an abbreviated format of an old English line. I always understood it to have come from "The Lord's Prayer", as in "Thy kingdom come, thy will be done on Earth as it is in Heaven."
However, I do agree with the recognition of the "Kingdom Come" mini-series as one of the best comic book stories of the decade. Every decade has its great comic book moment...the 80s had "The Dark Knight Returns"...the 90s had "Kingdom Come". This beautiful story (by Mark Waid)with just as beautiful artwork (by Alex Ross) totally set a standard for epic stories that has yet to be beaten. When I read the first issue, I thought "Oh, great...yet another story about an apocalyptic era in the near-future". However, as I moved on, it became quite clear to me that the story went beyond that. The fear in mere mortals of the current crop of "heroes" who believe and act like they're gods...the utter anarchy that results in these "new gods" callous actions...the attempt to put some order back in the system by the "old gods", led by Superman, which results in a precarious totalitarian overrule...the "lower gods" (ie. non-superpowered heroes), led by Batman, who speak for the mortals...all of these are strong themes that are explored in great detail throughout the story. I truly believe that this story should be recognized as a really great piece of literature of the 90s, surpassing many other literary books from that decade.
Finally, on an interesting note, there have always been rumours surrounding the possibility of a film version of "Kingdom Come". Apparently, the idea was explored at one time by Warner Brothers but it was deemed a logistically impossible story to film, given the number of characters that are involved. Who knows...maybe one day they will see otherwise? Until then, I'll make do with my trade paperback...which, by the way, includes an epilogue which didn't appear in the original print run of four isses.