was politically so stable over a period of more than 1000 years?
The political settings in the Republic were certainly modified during her history (mostly between the 9th and the 15th century), but overall the trend was always very smooth, and never too bloody.
It looks like that the Grand Council and the major families of the city were always well conscious that their interests were aligned (and subordinate) to the interests of the republic; and never seriously tried to rock the boat.
The last (and possibly the most serious) attempt to create a dinasty was with the Candiano doges in the 11th century; but I've the strong feeling that the Candiano were "allowed" to play their ducal role until the oligarchal consensus decided it was time to put a stop to them.
The infamous conspiracy headed by Marin Falier was a bit farcical, and mostly originated from issues of pique and pride rather than from ambition.
The Venetians were willing to enforce "committee rule", rather than putting their confidence in individuals. And their consitutional history shows that their main goal was constantly to put limitations on dogal powers.
Even if they ended up with a very complicated (and apparently absurd) system of checks and balances, the net result was that the republic lasted and prospered for a very long, long time (I doubt that another state lasted for so long, and with so minimal changes as the Most Serene Republic).
There was never an attempt to carry out a military coup. There was never a serious insurrection in her colonial possessions (ok, Crete revolted in the 14th century, but it was nothing very serious). Even after the 4th Crusade, and the aggrandizement of the colonial empire, Venetians went on as before, more or less.
Any thought? I even included "sheer luck" in the poll choices, but I cannot believe it. There must have been some peculiar venetian trait that acted as a catalyst.