When I refer to coherence, I am not saying that every single version of Batman is coherent with each other (they aren't). What I mean is that every single version is coherent with main idea of the character and the different interpretations writers have given about the Bat lore. You can get Morrison's BatGod, Rucka's detective, even funny Batman '66 and it is always Batman. It is always recognizable and coherent in its own way with the character. The strength of a cultural icon is measurable by how many different and convincing interpretations and versions you can have of it without betraying its original essence. In that regard, Batman belongs to the realm of immortal characters like Dracula and James Bond, way more than it can be considered similar to Superman. And I wouldn't be so focused on the fanbase, because IMHO Batman's appeal is strong for casual readers, too.
Fair enough. I will give you an answer. To me, Superman's history in the latest 20+ years is just an endless list of lost chances, with one exception - All-Star Superman (which is well, perfect and - just to be clear - is ONE possible interpretation of the character, in the same way that Dark Knight Returns is one possible version of Batman). It couldn't be defined in any different way. I won't indulge in the reasons why this happened: sometimes DC failed and sometimes fans didn't accept new interpretations of the character.
Please not that I am NOT nostalgic, because I don't think that when Superman was the peak of his success - in the 1950s and the 1960s - he was the best character ever created. It worked because those were different times: that version of the character was a very well-known component of pop culture, and it was appropriate for that era in the same way Batman'66 was appropriate for THAT era. The problem arose when after the 80s - because of a lot of different reasons - for these characters came the moment of evolving into something different and more modern. Batman succeeded, but Superman didn't - it's not that Byrne's era is without its merit, it's just that it was not strong enough to pave the way for more stories which had the same strength of Arkham Asylum or, I don't know, Night Cries. Superman simply missed the train. It's not that they didn't try to start something new and more complex later - during the Kelly/Loeb era (which is probably my favorite one, even if I see its limits), during the Johns era or even during the New52 era. But things always followed the same pattern - they started with very strong starting points, focused on one single aspect of the character (his aliennes, his moral doubts, his role), but they never - NEVER - developed this idea to the very end, in a coherent narrative which could finally "state" something new about the character and be a new starting point for something which could come later. Maybe it would be more precise to define it as an endless list of false starts. What I see now in the Superman universe is a group of completely disconnected elements, which taken singularly could be even potentially interesting but simply don't mesh well with each other. It's as if they were trying to build a porcelain vase by putting together the fragments from 50 vases which are completely different from each other. It doesn't make sense and there is no discernible direction, even if I think that Bendis is a talented writer (but there are several plot points in his run which don't make much sense, even if I appreciate the ideas).
What really frustrates me is that I see the potential of the character and its universe, I see that there are a tons of potentially interesting ideas which could put Superman in the same league with popular works like Doctor Who or Adventure Time or even Moebius' Arzach saga (with the right writer, of course). That's what I like - the potential which never comes to fruition.