h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Hmmm i think alot of the major prerequisites have been discussed correctly - education, fair courts, security
Here are a few that have been mentioned but not in strong detail (unless i missed it - there were a couple if so sorry)

Countries benefit greatly from at least a modicum of economic power - however there are other democratic countries so i would suggest being self-sufficent is the primary requirement.
Another thing that is needed is something to hold the people together while political parties form - for the first few year when democracy starts to form everyone is clumped together saying this is best but after that political factions start to form and a source of authority (either through power or prestige) must hold things together - but it most not be outside of the country or it would be rejected as an outsider.
Monarchies that are willing to function as part of a fair constitutional monarcy are one excellent source of this - another would be a man/woman or even a group of such that is respected by the people.
To wrap off my exceedingly long post i will say
The tree of liberty of water must be watered time to time by the bloody of patriots - but i would prefer to use the bloody of tyrants