I would probably turn that around and ask why are you asking?
Outside of command line editing features, some kind of completion, novelties, and prettiness, I think most of the time it always comes back to "I chose this shell due to syntax, or programming of some kind". e.g. tcsh due to the C-ness of it.
I am writing this from Linux, been using csh on Solaris and then bash on linux since, well since back then... I have seen a lot of shells come out, but every time I switch to one I think about my personal needs in the shell and portability (I do manage a large number of boxes, and most of them do not have extra items installed, including shells) so I end up back at bash/dash/sh.
Again, I would ask myself what you need in the shell. If ZSH or Fish or csh or ksh or some esoteric shell fit the bill, why not just try it as a daily driver? I mean, what is the worst possible outcome? (unless you make a living making life support shell driven software...)