This has always bothered me; I want to stress first that the Klingons as a fleshed out race are a brilliant addition to the Star Trek Universe, I love their honour bound warrior code and the various examples of this being abused in the various seasons to manipulate an Empire despite their strong codes of ethics... Regardless, in Star Trek Insurrection Riker & Deanna express disgust at the Son'a for subjugating and integrating two races into their society, despite the fact the Federation's greatest non-member alliance was with the Klingons from the early-mid 24th century.
With the Klingon's imperialist conquistador like attitude and the federations apparent example as a true liberal utopia, how could their relationship ever progress beyond basic niceties?