Mystery Probe

My writing group met last night. I mentioned that I was reading one of James Sallis's mysteries and that it seemed to me that the mystery genre allowed one a lot of leeway in process and subject matter. One of our female members, who is writing a mystery series now, and who has written romance novels before, said that she actually found mystery to be "more" confining in structure than romance. I believe she was talking about the need for a mystery to be more tightly plotted than a romance. That made sense to me, but then I realized that I'm attracted to "mysteries" in which the puzzle is secondary to an exploration of character and theme. In these kinds of mysteries, like the ones Jim Sallis writes, the plot is not tightly woven and there are a lot of sidetracks to follow. Although I've not read a lot of "mainstream" mysteries, I can imagine the rules for writing one would be more constraining than those for writing in many other genres. But mysteries also seem to allow for a lot of experimentation around the edges of the genre. I wonder if the same thing is true for other genres?