Rousseau thought that the more people have in common with each other, the more likely it is that they will work together for the common good. See The Social Contract. This is pretty much a tautology, but an interesting one.
If you are all small farmers, you are more likely to agree what you need and want from government than if ninety percent of you are landless peasants and ten percent large landowners. On its own, having in common that you are small farmers may be a more powerful uniting factor than other things that divide you, such as language or even religion. Switzerland got its act together despite having three languages (technically four, because a few people speak Romansch and they were and still are accommodated on the bank notes). It still has its act together and operates politically quite differently to the rest of Europe. Referenda form a regular part of political life. Governments are always coalitions. Offices rotate. Income is high. It has become less isolationist: it has joined the United Nations and, remarkably, the Schengen Area. The euro is becoming a second currency.
Rousseau would despair of the United Kingdom. How can you possibly get people with so little in common with each other to work together for the common good? An honest answer is that you can't. A significant part of government activity is directed towards getting people simply to be civil to each other. This has not been without its successes, even in Northern Ireland. We are less racist, less homophobic, less sexist than we used to be. Even the BNP seems more civil; some Muslim groups may now despise their neighbours more than the BNP despises them.
But the lowest common denominator of civility does not convert into the highest common factor of a community united for the common good.
Rousseau would almost certainly recommend that the United Kingdom be broken up. It's a failing state. Four independent states would have a better chance of welding together identities and a shared sense of purpose. England would still be too large and too fractured for Rousseau's comfort. He would suggest taking out London, which manages to have a strong identity despite - or because of - being the World in One City, in Ken Livingstone's felicitous phrase. There is something about very big cities which allows them to ride above the prejudices which defeat smaller communities.
I think Rousseau was thinking broadly on the right lines. I would vote for an English Parliament with powers equal to those of that in Scotland. But like Mr Salmond, I would want to pull out of the United Kingdom. It's had its day. Its an imperial relic. There are people who think of themselves as British, encouraged to do so by Conservative and Labour alike. More people think of themselves as English, Scottish or Welsh. No one thinks of themselves as United Kingdomish unless UKIP supporters do.
Unlike the English Democrats, however, I want to stay in Europe and join the euro.