What's common between the former 3 cases is an obvious escalation in the processes that lead to these radical changes. The Tunisian president simply chose to flee the country, the Egyptian dictator was forced to step down and surrender himself in the hands of justice (however you define 'justice' in Egypt), and Gaddafi was physically eliminated. The escalation still continues with Assad's fate hanging in the balance and Syria at the brink of a civil war.
The 5th case, Yemen, shows another tendency that also took shape after the Arab Spring. The negotiation process between rulers and opposition in Yemen is practically dictated by the Cooperation Council for the Arab States of the Gulf (CCASG), which is dominated by Saudi Arabia. It imposed a scenario of power transferring without serious violence and without physical threats for the president, who was granted immunity from prosecution, in exchange of him relinquishing power in favour of his deputies.
Here they are, the CCASG:
But here's something curious. The 6 CCASG states are monarchies. And not only did they manage to force a practically acceptable solution of Yemen's power-sharing problem, but those countries have demonstrated that in the Arab world it's considered dangerous to leave political decisions in the hands of factors and groups that are yet to assert their legitimacy.
So far not a single Arab king or emir has been seriously threatened with mass riots. The fact that all 6 monarchies on the Arab peninsula haven't been seriously affected by internal unrest (with the exception of Bahrain, where CCASG intervened resolutely and categorically), outlines one more peculiarity of the Middle East: the Arab monarchies (8 in total - Saudi Arabia, Kuwait, Qatar, UAE, Bahrain, Oman, Morocco & Jordan) rely on a particular form of legitimacy, which doesn't exist in the republican regimes of the region.
Granted, some dictators (most of them already gone into history), have tried preparing the ground for transferring power within the family while they were still alive. Mostly to their sons. And that made them even less popular in the eyes of their citizens. One of those sons, Bashar al-Assad, even managed to succeed his daddy at the reigns, and ain't going anywhere yet.
Meanwhile, it's fairly "normal" for a king to prepare their successor from within the family. What's more, their subjects expect that to happen.
Some republican dictators have tried turning power into family business - very often with the only purpose of material self-aggrandizement. And this way they gave even more ammo in the hands of their detractors and the opposition. But for the monarchs, being in power is considered family business, period.
The dictators tried re-shaping the very idea of republican government - they came either through violent coups, or via succession, or by the power of extraordinary "rights" granted by the ruling elites. Elections were turned into farce. The decades of being untouchable and increasing their grip on power to extents that by far exceed even the "legitimate" pretenses of the monarchs, were a serious factor that created extreme internal discontent which eventually exploded in the Arab revolutions.
Some Arab monarchs, like those in Jordan and Morocco, can also rely on religious legitimacy by claiming to be direct heirs of the prophet Muhammad. And the Saudi king carries the title of "Protector of the two Holy Mosques", i.e. he has his hands on two of the holiest places in Islam (Mecca and Medina).
The legitimacy of the monarchs is supported by the notion that the monarch (who's always "benevolent", you see) is capable of doing reforms much easier than a president or a prime-minister, and without worrying that those reforms would backfire and shake his throne. In principle, the monarch is in a way detached from everyday governing, he has delegated some limited rights to a government which focuses most popular discontent on itself, and which is easily expendable and could be sacrificed without directly threatening the monarchy.
Let me make it clear that I'm in no way arguing in favour (or against) monarchies, especially of the Middle East type. But it's curious to think about this: turns out that the legitimacy of the monarch gives him a great credit for potential reforms, which after all was the end purpose of the Arab revolutions. Ironically, the total power of the monarchs has ultimately allowed them to be more flexible in this situation.
In each of the Arab monarchies, even before the actual eruption of the Arab spring, some hints for reform were noticed, be it real or impending. The king of Jordan Abdullah II created two commissions - one for the "national dialogue" (main task: to forge a new election law), and another for Constitution amendments (it has already introduced a plan of 42 changes in the Jordanian constitution). Moreover, the monarchy is not being attacked in Jordan - most of the discontent is directed at the necessity for changes within the regime itself, in the way of governing... all the drama is not about removing the monarchy or getting rid of the monarch.
Things are very similar in Morocco. In Saudi Arabia, the king Abdullah has praised his subjects for their reluctance to protest, but he also hurried to take some serious economic measures to limit the sense of unjust distribution of the national treasures. A $130 bn program was introduced that would create new jobs, and new subsidies and financial aid for the Saudi citizens in distress was accepted. Similar events happened in Oman, where sultan Qaboos responded to rising criticism from his subjects by making some reforms, repairing his government and dispensing a massive relief subsidy.
The CCASG countries have invited Morocco and Jordan to join the organisation. The explanation: this could stimulate investment in the member countries. Meanwhile, Saudi Arabia promised that it'll be paying $1 bn annually to both Oman and Bahrain, which is quite a sum for those two tiny countries. It looks like the Arab monarchies are united and support each other in such moments, and at least for the time being they're able to preemptively react to potential factors that could cause mass unrest among their subjects.
This tendency is showing. Despite their conservatism, the Arab monarchs seem to realise that they live in a dynamic world and the changes will sooner or later start affecting them. For now they're able to run ahead of the events with decisions that could be easily taken individually, by the monarch, with little to no political risk. And they've made steps that are strengthening their allied relations and demonstrating an aspiration to establish their "club" as a regional superpower, that would serve as a geopolitical counter-weight to another regional power - Iran. Because it's mostly about Iran.
I mean just think about it. CCASG + Jordan are located in a region where Iran is consistently using the Shia communities and some of the more radical Sunni groups as a "fifth pillar" for exporting political influence abroad. The removal of Mubarak's regime in Egypt has deprived Saudi Arabia of a very important Sunni ally against Iran's ambitions. There are problems with the Shia community not just in Saudi Arabia itself, but in the rest of the CCASG countries. Saudi Arabia is having the South Yemen problem to deal with, the place now practically becoming a headquarters of Al Qaeda (and Al Qaeda has declared Saudi Arabia as its primary enemy, along with the US, the West, and Israel). Basically, Al Qaeda's ideology (and the hatred for the Saudi regime that originates from the time of Osama bin Laden), is rooted in the early years of the Egypt-based Muslim Brotherhood. And that group is about to take the reigns in the biggest Arab country, via its political wing and through legitimate political means, namely: popular elections.
Saudi Arabia was also successfully pushed away from Lebanon as well, where their staunch ally Rafic Hariri used to be prime-minister. But now Lebanon is in the hands of the Iran-backed and Syria-funded Islamist organisation Hezbollah. Currently Iran is Syria's only real ally, and that might partially explain the events in Syria. The political elite in Damascus almost entirely consists of Alawites, who in turn are a sect of the Shia branch of Islam. If Assad's regime survives (and at least for now it's holding its ground), the big winner would again be Iran, and the big loser - Saudi Arabia. Iran is advancing, and CCASG are trying to find ways to respond. The US relocation of forces from Iraq into other parts of the Gulf means that the US is still part of the game, and it's clear whose side they're on. In this context, the recent events with Iran flexing muscles in the strategic Strait of Hormuz is no coincidence, either.
In Iraq the Shia majority who were oppressed by Saddam's regime, are now dominating the political landscape. With the withdrawal of the US troops from Iraq, Iran could turn from mostly an observer sitting in the sidelines to the main source of external influence. And it's already happening. The paradox here is that by removing Saddam's regime, the US has accelerated the spreading of Iran's geopolitical influence.
The combination of the Muslim Brotherhood taking over in Egypt and the military striking a deal with them + Assad's regime potentially surviving in Syria + the widening of the Iranian influence in Iraq, means that a kind of an "arc" is taking shape, spreading from Western Afghanistan all over to the Mediterranean. This creates potential conditions for a new regional alliance centered in Tehran, which would border on Jordan and Saudi Arabia by land, and on all the remaining CCASG monarchies by sea. In these conditions, the confrontation between the moderately pro-West monarchies and the staunchly anti-West Iran-Syria axis now seems imminent. Now, more than ever, the Middle East looks like a powder keg waiting for that tiny sparkle. Whether it'll be a false-flag "provocation" or another "revolution" sponsored by either of the sides, the fuse is about to be lit anytime now.