"If you want a picture of the future, imagine a boot stamping on a human face—forever."
George Orwell (Eric Arthur Blair), Nineteen Eighty-Four
Orwell's Nineteen Eighty-Four is brilliant, terrifying and useful. It's been at its best fighting against governmental intrusions, and is often quoted by journalists and even judges. It's cultural impact has been immense. And, hey, it's well written.
But that doesn't mean it's accurate as a source of predictions or counterfactuals. Orwell's belief that "British democracy as it existed before 1939 would not survive the war" was wrong. Nineteen Eighty-Four did not predict the future course of communism. There is no evidence that anything like the world he envisaged could (or will) happen. Which isn't the same as saying that it couldn't, but we do require some evidence before accepting Orwell's world as realistic.
Yet from this book, a lot of implicit assumptions have seeped into our consciousness. The most important one (shared with many other dystopian novels) is that dictatorships are stable forms of government. Note the "forever" in the quote above - the society Orwell warned about would never change, never improve, never transform. In several conversations (about future governments, for instance), I've heard - and made - the argument that a dictatorship was inevitable, because it's an absorbing state. Democracies can come become dictatorships, but dictatorships (barring revolutions) will endure for good. And so the idea is that if revolutions become impossible (because of ubiquitous surveillance, for instance), then we're stuck with Big Brother for life, and for our children's children'c children's lives.
But thinking about this in the context of history, this doesn't seem credible. The most stable forms of government are democracies and monarchies; nothing else endures that long. And laying revolutions aside, there have been plenty of examples of even quite nasty governments improving themselves. Robespierre was deposed from within his own government - and so the Terror, for all its bloodshed, didn't even last a full year. The worse excesses of Stalinism ended with Stalin. Gorbachev voluntarily opened up his regime (to a certain extent). Mao would excoriate the China of today. Britain's leaders in the 19th and 20th century gradually opened up the franchise, without ever coming close to being deposed by force of arms. The dictatorships of Latin America have mostly fallen to democracies (though revolutions played a larger role there). Looking over the course of recent history, I see very little evidence the dictatorships have much lasting power at all - or that they are incapable of drastic internal change and even improvements.
Now, caveats abound. The future won't be like the past - maybe an Orwellian dictatorship will become possible with advanced surveillance technologies. Maybe a world government won't see any neighbouring government doing a better job, and feel compelled to match it by improving lot of its citizens. Maybe the threat of revolution remains necessary, even if revolts don't actually happen.
Still, we should refrain from assuming that dictatorships, whether party or individual, are somehow the default state, and conduct a much more evidence-based analysis of the matter.
There are both ideological and practical differences. A monarch is head of state explicitly because of his ancestry; further, they tend to go with a hereditary aristocracy explicitly identified as such. (The Soviet nomenklatura was certainly hereditary, but it was not part of the public organisation of the USSR.) A dictator may inherit power from his father, but that's not the basis of his legitimacy; he is dictator because he "safeguards the revolution" or "leads the Party" or whatever. In fact, monarchies generally speaking don't have an explicit ideology, unless you count "family X shall retain the throne"; dictatorships have,a least publicly, some sort of theoretical underpinning, whether it's marxist-leninism, lebensraum for the X race, or Ordnung Muss Sein.
Further, monarchs have, as a general rule, been less obvious about the mailed fist; secret police and censorship certainly occur, but they are not central, obvious features of the regime. They also tend to be less overtly militaristic; note for example that Britain has a Royal Navy and a Royal Air Force, but a British Army. That's because it descends from the New Model Army, raised by Cromwell to fight for Parliament (against the King) in the Civil War.
Monarchies are also, perhaps, more long-term in their extraction of surplus; shearing the sheep rather than butchering it. A warlord or bandit who expects to be thrown out next year has no incentive to let the peasants keep their seed corn; a monarch who expects to pass his throne to his children will even let them keep enough of their stuff to do long-term improvements like, say, building roads and canals. A dictatorship, I suggest, is usually somewhere between these two extremes. Henry VIII's dissolution of the monarchies was unusual in a monarchy, but would not stand out in a dictatorship.
ETA: sorry, mixed you up with other commenters. Here's what's left of my response:
The original claim was that dictatorships are less stable than monarchies. Your definition of the difference between the two might say that this is because monarchies have stable inheritance rules, while in dictatorships, which don't, a new dictator is often weak at first. This can topple or amend the dictatorship.
This claim (which you didn't make) doesn't convince me, to which I responded (after correcting my comments) in my reply here.