The characters are of secondary importance here. They have to move in a certain way to be at a particular place at a particular time to drive the plot forward. So they cannot be independent or fully realized.

Plot is everything in a locked room mystery. This is not something that Johnny or Sue Writer takes on. This requires an idea and a plausible way of explaining the locked room. Only a few writers typically use this type of plot as it requires such an eye to details.

Indeed, the mystery runs high in these books. The mystery needs to be solved in order to move on. The desire to solve it is overwhelming.

There can be some suspense if other events depend on a solution. A character may go to jail or the electric chair if the real killer isn't revealed, but it's not the key element of the book.

The crimes are usually more personal ones, and therefore there's not much in the way of thriller here.

By definition, an impossible crime has an element of strangeness to it. The surreal qualities of a crime that couldn't' have happened open up the possibility for strangeness.

Typically not as much violence, because at heart this is a cerebral crime. The characters are thinking about how to find a solution to the problem.

Typically not as much action, because at heart this is a cerebral crime. There's more discussion than action.

The locked room subgenre definitely falls into the mystery genre. It's a puzzle plot at its best. There can be some elements of thriller or suspense, based on the book, but it comes in second to learning how the crime was committed. Given that the plot is foremost, it's not likely to fall into the crime novel category either.