So, we are supposed to believe that species like the Klingons and Romulans are a significant threat when first encountered in the 22nd century, and stay so throughout the 23rd and 24th century.
But neither are they at any point in time so strong they can overpower the Federation without too much effort (well perhaps the Klingons could have with pre-Federation earth), or they would probably have done so, so they can't have had a huge headstart either.
This all while the Federation is supposed to make huge strides in that timespan, and combines the efforts, resources, economies and brightest minds and perspectives of an ever-growing number of species. So, somehow the Klingons and Romulans must have kept up. But how can mono-species empires (or, at best empires with one single ruling class) hope to compete with them, long term? Subjugation would be far less efficient than voluntary cooperation.
Where do they get their scientist and economies, if huge amounts of what they could generate is spilled under a warrior caste (or stifled under a senate and secret police that tightly controls society)?
EDIT: can't seem to get rid of these annoying HTML-codes for which I apologise (not even deletion of entire paragraphs seems to help (and can't delete this opening post) ).