Perhaps Rome became doomed (in the barbarian department anyway) when they stopped splitting up "barbarian" settlers like the Goths under Alaric, Burgundians under Gundobad ( i think) and Franks under Clovis (i probably i have the name wrong, i can't remember who origionally lead the Franks into the empire). Also anti barbarianism (eg. Stilicho's case) probably played it's part in alienating "barbarians" within the Empire (like the goths who joined Alaric) and putting them off Romanisation, why become like those that hate you?
Plus ofcourse by the time of the fall many many "barbarians" had nice shiny roman equipment from victories on the battle field and serving in the army( as auxilia and later foderati) since the empire's inception.
Hope this is clear enough to get my point across, that Rome created formidable enemies.

I realise there was a multitude of other problems and reasons