Well, I've brought this thread back from the dead it seems.
I chose economic decline. While any of the factors taken individually, couldn't bring down the Empire, when all combined into a perfect storm, spelled doom for the Western Empire. I picked economic decline because over time, it left the Empire in a weakened state. A wealthy Empire should not have had to strip troops from the border regions on the Rhine to fight the Goths in Italy as Stilicho did (or tell Britannia to defend themselves as best they could). They simply couldn't field enough forces to defend themselves.
And the economic decline itself is the result of several factors. The Roman economic machine was driven by plunder. The minute Rome stopped expanding, the booty stopped flowing into the economy. Moronic acts by Emperors like doubling the legions' pay didn't help matters, or staging lavish games, or building churches with state money. Disease is also a factor in the poor economy. Tax payers dying because of plague isn't going to help your economic base and we see a depopulation in the Empire in the 3rd century because of disease. Crippling taxes only stifles the economy and encourages tax evasion and barter. I think at some level, the Roman leadership understood basic economics, but there was too much pressure to simply raise taxes or mint more money. The quick answer was usually the one chosen because Emperors with the mob and hostile legions at their throats have to take quick action, so just mint more coins.
Civil Wars existed all throughout Roman history, so that wasn't much of a change, but in the weakened state of their economy in the 5th century, they just couldn't afford the luxury of fighting each other. But why change? It was the Roman way. They had been conducting business like this for 1000 years. I don't think they realized that the Empire could really fall if they continued in the same manner.