Which brings up another question, if TOS was a groundbreaking show, why did the groundbreaking stop with TOS? I admit to not having watched the other series (save a few Next Generation episodes), but I've never heard of anything groundbreaking as far as any of the following series goes. Did being revolutionary become less important after TOS?
Well you had TNG's The Outcast
and DS9's Rejoined
, which attempted to tackle gay relationships in an underhanded fashion.
But by the time TNG came around it was more a franchise and The Powers That Be were more interested in not damaging the brand than being groundbreaking.