Response to the above:
1) When a Empire goes from a single city to a world dominating empire it has to do with more than luck. It is because they have developed a culture, military and economic system that allows them to do so. Can't buy the luck theory.
2) There is some merit to the second theory, but it is not military alone that allows a military to stand -- a professional army is not possible without a political system and economic system to back it. The Republic in the early days of Rome was about the best interests of the republic and produced people that had to rise to power through competition -- this made them great people that wanted to do great things for the state. I am not sure about location because there were others in similar situations. The fact is Rome had all the luxuries of being a sea town without being on the sea thanks to its rivers
3) Not sure about this theory. The Romans definitely had better tactics and well trained troops, but technology at times was slow nad they never really developed calvary to where it needed to be. They had their own culture, but accepted others but i don't see the adaptation with every culture. When they saw something better they sometimes adopted it, but other times they did not.
4) Not always but they were rather accepting to a point. They had their own culture they beleive to be superior as well.
5) Between 2 and this one I think you have the answer. The republic produced very capable individuals who lead with brilliance. Cato, Scipio Africanus, etc. etc. the list gets very long becasue it was these men who lead their people to victory time and time again.
I think Rome developed a political and economical system that could support a standing army that was lead by competitive but patriotic men. In such cases you are going to be tough to beat. You are also going to build something others can build on later.