Yeah one thing that I liked about City of Heroes and Champs Online is that once in a while, it includes a non-combat mission in its main storyarc. The one time players are given a diplomatic mission in the main storyline and what happens? 2,800 Jem'Hadar crashes the party and suddenly its all combat again.
Yes there are diplomacy missions in STO, but players have either to stumble upon them by accident (or by curiosity), or they are inaccessible until the player gains a certain rank in diplomacy.