What are the main differences between road saddles and mountain saddles? There are thousands of saddles out there. Some are designated as road saddles while others are mountain saddles. However, the marketing copy does not seem to describe the difference between the two types, even between two similar models (for example, Selle Italia seems to have an SLR for road bikes and an SLR XC for mountain bikes).

In general, what are the main differences between road saddles and mountain saddles? What, if any, are the potential downsides of using a road saddle off-road or a mountain saddle on pavement?
There is a lot of variation across models of saddles even if you stay within one genre (eg only consider road or only consider MTB).

However, you'll find if you look at popular racing saddles that MTB saddles tend to be shorter. This is because in singletrack you're very frequently moving onto and off-of the saddle. A longer saddle might interfere with the "body-english" that some MTB riders use.

That said, if a saddle fits, it fits. That is the most important criteria. Everything else is a distant secondary consideration.