To be fair, the OP has a point. Classes have no DEFINING characteristic which makes one more useful than the other for a particular type of content. The mechanics might change, but it's just window dressing, and has no real impact on gameplay. In my opinion, the only exception to this rule is tanks, where Paladin still reigns supreme in high end content simply because of the crazy damage that a warrior would take in the same situation, putting too much stress on the healers.

Still, one exception does not a rule make and it would be nice if classes had unique abilities which make them truly stand out from the crowd. Some would argue it would promote excluding certain classes from content, I would argue it would promote a wider range of classes being invited.