1. Coastal position allowed Genoa and Venice to be not only hubs for inland trade, but also naval trade.
Naval trade allowed transportation of goods from far away, as it was faster and more safe.
2. People of Italian trade cities probably borrowed some words, clothes, cuisine and inventions from those who visited them.
3. Signoria was a type of government, were power was held by one dynasty, or party. Signorias were often established in times of crisis, when it was important to have a strong figure, who could make swift decisions to stave off the threat.
4. Signoria helped to create strong civil government, relatively independent from pope or monarchs. Because of it italian cities could be more open minded towards science and secular culture.
5. It's a situation which is very prone to corruption, especially nepotism. The ruling family would try to perserve status quo, and would offer most important offices to familiy's members. The quality of people in power would deteriorate, as they are appointed not by merits and skills, but by blood.
It would also be important to be in good relations with the ruling family, so it would probably be necessary to give them bribes or gifts.
You have much higher chances to receive an answer when you divide this question into 2,3 or even 5 shorter questions :)
