The secret good ending Lancer-X mentioned is actually due to a small bug in the game - start on the 6th loop, choose each daughter an equal number of times, choose Maya when the maid choice appears, then a new, very interesting conversation with Gumoruku takes place - the game then returns to Maya’s path/ending except it doesn’t loop when it probably should. 
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In addition to what Lancer-X mentioned about the dystopic world view, the other strengths of Mugen Kairou are the quality of the writing (not implying that the other major Black Cyc titles, particularly EXTRAVAGANZA are badly written, but the text in MK is (almost?) on the level of Setoguchi Renya’s eroges) and the nature of the scenario. Along with being an excellent story, it also a has a strong philosophical bent to it, as well presenting a very cynical satire/view of modern society (incidentally, the latter is also definitely the case for the Rasen Kairou games). It’s definitely an issue of personal taste, but as far as dark eroges go, I prefer deep, introspective (depressing) stories (Tenina, CARNIVAL etc.) to action based eroges (MinDead and the like).
Overall, I’d say that the characters have ‘improved’: Nanaka isn’t as ‘endearing’ judging by the demo of 2 as she was in 1, but the new second daughter, Yurie >>> Reika (although the demo indirectly reveals that Reika is in the game in the maid’s role, ie as Hana) and Kaoruko as the 4th daughter has become a full-fledged tsundere and thus >>> than Nanaka in MK1 :P.
And finally, only in Black Cyc eroges do male characters win popularity polls (happened in Gun Katana as well, and although this was only the pre-release one, I’d be very surprised if Taro doesn’t win the real poll as well).
