The Klingon Empire can provide about as much story as the Federation. Just play the ages old Star Fleet Command series, and you can see what fascinating campaigns they created for Federation, Klingons, Romulans - in a way the ages old game Star Fleet Command III has more and better story / content that Star Trek Online has today.
Even Guild Wars did not managed to entice the majority of players to play too much PvP before the more casual and larger scale pvp modes came into play. Small arena pvp is not enough to keep a lot of people entertained for ages, and for sure not in a subscription based game.
Absolutely agree on that one, there were even whole games based on Klingon content (Honor Guard, Klingon, etc) not to mention the abundance of episodes (mostly DS9) which give a fairly good lookout on what a Klingon might do besides kicking Feds around.