It's fall was composed of both internal and external factors. The external factors were primarily invasions from Germanic/nomadic tribes floating around in the Russia and northern Europe area (which included Attila the Hun! He was a big one :) The Vandals and Goths and I think there was another, but I'm failing to remember the name exactly.)
Internal factor included lack of social/political responsibility. The ground on which Rome had gotten it's strength was disappearing as the morals declined through gladiatorial games and people were focused a lot on themselves and self pleasure. (This focus on self pleasure and lack of responsibility in both men and women was shown through the adoption/accepting of homosexuality in men, the increase in women prostitution for the polytheistic practice and the population decline as fewer and fewer had kids.)  
Also, the Roman empire stopped expanding after a while. With no continuous expansion, the economy slowed down. Now, think of how much money it takes to run and entire empire :P The Roman empire was also, for a while, paying off some of the tribes that wanted to invade because Rome was worried they'd lose if the nomadic tribes ever did.
So, the Roman empire began taxing the people heavily. This wasn't a great idea because then people didn't want to start their own businesses due the the high chance they wouldn't succeed with the lack of currency movement in the empire at that time. (If you just thought of the Great Depression when you read lack of currency transitioning, you are on the right track. :) I could try to explain... look into business class.  And interestingly enough Pres.Bush of the USA was seen trying to take so,me taxes off because he didn't want entrepreneurship to decline like the Roman empire)
 Anyway, some argue that because Constantine also adopted Christianity as the official religion, that made people not willing to fight and take part in their duty as Roman citizens. The decline of the Roman military was a big reason! With fewer and fewer Roman citizens around - because of population decline due to people not having babies -, the Roman empire had to start hiring germaic people around the place to work in the army. This was not a good plan. For what reason would they fight for the Romans?
There was little or no allegiance! Only a paycheck...
 Anyway, It was all pretty bad and ugly and awful. Some compare how the USA is focused on the Games - football, Olympics etc - and morality decline (I'm not taking one side or another in here since it's up to you what to think) and birth control/adoption with the Roman empire. They lasted a little more than 1000 years... The USA has only been going at it for 238/239 years. Will we make it past them? 
Sorry for the long reading :) I'm a geek about history.