LightSpectra said:
As for rivers: (1) they're useful for travel, (2) they serve as a gigantic moat-wall, (3) having a source of fresh water is great (plus it means you can generally avoid aqueducts/water systems for the pre-modern era), (4) a large surplus of water generally allows for a more hygienic and larger population, (5) rivers can be used as a power source, etc.
There's some problems with your assumptions: (1) rivers are not in the whole useful for travel from point A to B travel, in most cases it was slower than using the comparable land route and sea travel is in most cases better served by a harbor rather than a river egress; it was however useful for trade specifically the movement of victuals -- food and drink -- whether from the hinterlands or the sea and other bulk goods. The transportation costs of river or sea trade for bulk goods in almost all cases is exponentially cheaper than land travel. The ability to provide lots of food cheaply with some degree of reliability -- rivers don't dry up for instance -- would in most cases imply a larger population, which in-turn implies a larger tax-base, which in-turn implies a stronger power-base and so-on. If I were to look at the characteristics of capital cities in history, I would expect generally to find them to be the most populous city in the nation. That would help to explain why capital cities are generally cited on rivers or in the absence of rivers near the coast.
(2) I don't think that was a serious concern of the rulers, the value of a river in defensive terms generally declines over-time as the city sprawls out and more bridges are constructed. It may have had something to do with the selection of the site in the first place, but any defensive considerations were probably minimal by the time most of the European states were becoming recognizable.
(3) Except more than a few are built at the confluence of the coast and the river which makes the water rather salty -- London, Copenhagen, Lisbon etc. It also has the same kind of declining utility as (2) we know, for instance, that even early on many rivers were undrinkable because of the effluent both human, animal and industrial that was released into the water -- tanneries and dyers workshops were known to be capable of fouling the water visibly for kilometers and even past that point drinking the water was probably inadvisable!
(4) Same problems as (3). Population often existed in-spite of the rivers, drawing water from wells and springs.
(5) That wasn't a consideration, diverting even a portion of a large river for use in a water mill or fulling mill etc. didn't come into being until long after many of these cities had taken on 'national significance'
LightSpectra said:
Most of the Italian states really didn't have much going for them beyond their one big city, so they were more or less forced to locate their government there.
That's completely the wrong way around. They weren't forced, in most cases, the civil governments of the cities were already existent long before they formed 'states'. It was these city governments that moved out and exercised their power, and not the other way around like most of Europe.
Jeelen said:
That's not typically European, but rather has to do with the importance of rivers; historically rivers are a good spot to start a city (sound familiar?). That such a city might eventually become capital to a nation (while most cities do not) depends on other circumstances.
In other words: what made a good capital? And why was access to the river and the sea so important to the selection of that capital?