Is there anything special about a focal ratio of 6?

An f6 is very popular because it combines the best of features that amateur astronomers generally like. It allows a decent degree of magnification, and it is a "fast" enough focal length to bring out faint nebulous objects. It is also less likely to have aberrations that might be more common in shorter focal lengths. It's not some magical value, but it is a very good combination of features. It's a sort of "one size fits all" thing. As a consequence, it is not the ideal f# for any specific type of observing or photography.