And you know what? They really aren't true fans.
Without a doubt, the human condition lies at the very heart of Gene Roddenberry's Star Trek series. Ubiquitous to all of humanity is conflict, which is driven by everything from love to hate; and especially by ignorance. Thus, combat makes for perpetually enthralling television as well as video games.
I'm not here to defend Star Trek Online as the perfect MMO because it hasn't been released yet. However, I can speak in defense of Cryptic's choice to pit the Federation and Klingon Empire against one another. Why? First of all, it's complete adherence to Trek lore. Kirk hated them, Worf struggled to understand them and even Picard -- the great diplomat, but more on that later -- had his troubles with the almost villainous Klingons.
As someone who was introduced to the Roddenberry's universe through TNG, I understand why some people may feel like Trek is less about combat and more about diplomacy. Picard was very much the diplomatic captain, although his encounters with the Borg hardened him into more of a soldier than he (and some fans, most likely) would have preferred. However, TNG is representative of only one aspect of the series.
James T. Kirk was quick to fight, especially if it was a Klingon looking for a tussle. One of the most memorable scenes from The Original Series is the battle between Spock and Kirk in the episode Amok Time. Many of the movies featured several battle scenes ranging from ship-to-ship to between a few characters and as film technology allowed grander special effects they only became more prevalent.
With TNG, while there were Q episodes and much talking due to Patrick Stewart's background in stage acting (he's never not interesting to listen to) the series became darker and more combat focused as time went on -- especially once the Borg were introduced. As for the movies, well, First contact most likely inspired some of the massive Borg Cube battles we've seen in Star Trek Online trailers.
Next in line is Deep Space Nine, which was a series about a father and son coming to an oppressive alien space station and butting heads with every species in the known galaxy. Eventually, Sisko got a hold of the Defiant... and well, then things got even more combative. Why, by the end of the series DS9 became known for some of the best shoot-outs and space battles in the entire series. Nobody messed with 'The Sisko' and walked away without at least a limp in their step.
Then we come to Star Trek: Voyager, which was without a doubt the heaviest science fiction influenced of all the shows. Even still, the crew of Voyager constantly dealt with the Borg (on their home turf, no less) and many more aggressive opponents in battles. By this time, CGI had grown to a point where some of the most exciting moments in Voyager were its battles -- although I've always had an affinity for the holographic doctor scenes, myself.