There are two quite interesting factors in the decline of the Roman Empire that no ones mentioned yet. Sure, they're not magic-bullet-it-was-definitely-this type stuff, but still:
1.Lead. Good old Pb. The Romans were crazy or it. Lead pipes, lead cosmetics, lead based paint. More to the point, they also flavored their wine with it. Lead poisoning can have some really horrible effects: abdominal pain, confusion, headache, anemia, irritability, and in severe cases seizures, coma, and death. Not to mention what it does to developing fetuses. The fact that the richer you were the more exposed to lead you were would mean that the effects would be most prominent in the ruling classes. While not really a definite factor in the collapse of the empire, it can't have been a good thing.
2.The shift in Legion training, tactics and equipment in later periods. Usually, when we're talking about the glorious pwning of everyone else by the Romans we mean the time after the reforms of Gaius Marius (who basically switched Legion recruitment policy from "Are you a citizen? Got some Land? Got armor? Got a sword, shield and three spears? You're in." to "I don't care how poor or foreign you are, join the Legion for 25 years, get paid a good steady wage + all the loot you can plunder and get citizenship and a land pension at the end of it all") but when Rome was still expanding. During this period Legions were usually composed of 5500 legionaries plus about 500 auxiliaries, usually foreign mercenary cavalry or archers, often recruited form local enemies of whoever they were fighting at the time. At this time legionary armor was relatively heavy and offered excellent protection (I can't remember the latin name of it off the top of my head, but I'm thinking of the type of banded legionary armor that became popular in the Late Republic/ Early Empire because it was cheaper and quicker to produce than chainmail). Later, when everything was falling apart, the Empire had been split into East and West and the barbarian invasions were on the increase the setup was different. Legions weren't composed of 5,500 heavy infantry plus appropriate support troops but 1000 infantry in much lighter armor. The reason for the switch to much lighter armor was both financial (thanks to the shrinking revenues of a collapsing empire no longer being topped up by mad war lootz) and ergonomic. The later era legionaries, lacking the iron hard physical training of their forbears, absolutely hated the idea of having to wear really heavy armor all the time (30-35lbs).