There was a time that I really enjoyed art capers, art mysteries, books about art heists, and so on. But over the last year, I've read three of them (all published within the last five years or so), and I didn't care for any of them. The writing was fine, the plots standard, but nothing to really excite me.
I've also read a bunch of art manuscripts over the last couple of years and passed on all of them. Looking at this now, I think I requested them more out of a "remembrance of things past" than because it's a subgenre I still love. In fact, I like steampunk a lot more... it seems somehow fresher and more adventurous, while those art manuscripts (and published books) seem like retreads.
Somehow I think The Da Vinci Code is responsible for this. It was a category killer; given its success and ubiquity, everything else is going to pale in comparison. I read it, suspended disbelief, and enjoyed it; but I suppose that reading it was like going parachuting, which my wife and I did for our tenth anniversary. I enjoyed it, was glad I did it, but do not feel the need to ever do it again.
It's almost like the pure police procedural; I think that's a subgrenre that's losing (or has lost) ground, too. I almost never see manuscripts about a police squad or detective squad; most of what I get is single sleuths (sometimes within the context of going rogue within their department - but in this case, the department is composed of antagonists, not comrades in arms).
Usually I can identify trends in what I'm seeing, but the last year or so has seen some real oddities cross my desk. The agents are taking on some truly bizarre things, I think because so many of them now have other options if they can't get a contract for the manuscript - like taking a portion of the writer's royalties even if the writer ends up self-publishing, because the agent has provided representation. I guess the one trend I can identify is the Gone Girl clones that are still coming my way; as if that phenomenon could ever be duplicated.