Personally I think everyone is selling the Klingons far too short, especially given the amount of disparate personalities, not to mention evolution of certain characters that we've seen over the many years of onscreen Trek.
Now I'm not saying that the Klingons should join the Federation. Just that the prospect isn't that far-fetched.
From the outside, yes, the Empire appears to be nothing more than a collection of space vikings/barbarians - but surely such a society would be hard-pressed to maintain an interstellar empire as long as it supposedly has? There's got to be much more going on beneath the surface than just the instinct to fight everyone.
Consider the revelations of Enterprise's Judgement, where we learn of the existence of other Klingon castes besides the straight-up warriors. And how their culture had been undergoing a shift towards a more warmongering staunch for some time by that point.
Consider the shift between Enterprise and TOS, where the Klingons appeared far more regimented and disciplined in their approach to combat than the warriors of the previous generation, or The Next Generation.
Clearly Klingon society as a whole is fluid and capable of change, or regression, as much as any human society. And while their society may force the foul-smelling barbarian mould on many of its citizen, it is by no means intrinsically in their nature. Case in point; Worf, a man raised outside Klingon culture yet deeply entrenched in it, who does his best to balance the concept of honour and loyalty he learnt from humans alongside his people's same concepts. Obviously to varying results.
And of course with Worf we were given Martok; a commoner, supposedly forbidden from joining the warrior class (Caste?), who rises through the ranks to general and then Chancellor, a man who escaped a certain death in a Dominion prison camp, a man who knows how fickle fate can be. I believe he's displayed a tolerance for other cultures and beings rarely seen outside of Worf. When challenged by Nog to get a handle on his troops, he respects the Ferengi's courage, whereas you'd expect any normal background barbarian Klingon to simply stab him for disrespecting his honour - but Martok isn't like that, he knows simply swooping into battle at a moments notice isn't everything.
He is Klingon to the core and won't apologise for it, however, he has a respect for other species rarely seen in cliche warriors. Another interesting example I like is What You Leave Behind, during the toast on Cardassia Prime with himself, Captain Sisko and Admiral Ross. Martok is revelling in victory, but Ross and Sisko can't get over all the death around them and simply pour their drinks out. Is Martok outraged? Nope, he shrugs and mutters about humans having no stomach for bloodshed (or something; the line was Klingon, not my first language

). He's not going to force the issue the way a typical Klingon might.
My point being; we've seen the capacity of Klingon characters to evolve, change, act outside of their species so-called character, and we know their society is capable of change.
Is it so far-fetched that Chancellor Martok, or his successor, could maybe introduce a series of reforms elevating the status of the other Klingon castes? Perhaps eventually see a rebirth of the empire as a more egalitarian society? Maybe the Klingon Defence Force could be re-purposed as the Federation Defence Force, creating a complimentary service to Starfleet, while still allowing the warrior class to exercise their martial prowess when the need arises?
Just spit-balling there at the end, back to the point: The Federation is about embracing diversity, and I think the Klingons have that in spades, we just haven't seen as much of it as we have the Klingon military.
And if it's possible the Klingons could one day join the Federation, then anyone can.
