Lately I've been reading The Roman Emperors by Michael Grant, and one of the things that struck me was how pathetically small most of Rome's emperors were. They were utterly pygmy; I could count the number of actually good, worthwhile rulers on my hands. The system may well have collapsed from the moment of its inception if not for the stewardship of Augustus. Things only got worse as the empire aged and its need for strong leadership grew: Honorius in particular strikes me as among the most flagrantly worthless men in recorded history, and I am convinced that his reign marked the point of no return beyond which nothing short of divine intervention could salvage the empire.
With serious reforms and competent military leadership, the empire may well have survived the economic stagnation and barbarian invasions that ultimately wrecked it (Diocletian's reforms may well have allowed the empire to survive another century). But without a capable emperor to do so -- and by the end, there never was one -- Rome had no hope of continued survival in the face of the hardships that rocked her.
I'm not too terribly familiar with this theory, but it's the most plausible one I've seen to date. It seems reasonable that the handful of good emperors may simply have not been enough to withstand the damge wrought by all the bad ones.