A team is only as strong as its weakest link.
Look at the performances of the bowling attacks all series. Siddle carried the Aussie attack at Brisbane, they were all dire at Adelaide, Johnson and Harris wrecked us at Perth, and only Siddle again stood up at Melbourne. Australia's bowling in virtually every innings has relied on one man being the main wrecking ball - Siddle twice, Johnson and Harris have all claimed 6-for in the series.
Whereas with the exception of the first innings of Brisbane, England have consistently dismissed Australia cheaply without any of their bowlers having a really outstanding series. It's been the theme all tour from the first tour match - all of the bowlers are chipping in to contribute without relying on any one of them pulling a magic spell out of their arse - only Finn, Swann and Tremlett have taken a Michelle.
As PEWS said, the only weak link in the side is Collingwood's batting, and even then, what Collingwood brings in the field to a degree compensates for his batting failures. Compare England to India - there's question marks over both openers in conditions where the ball does a bit, Dravid may or may not be over the hill, Ganguly hasn't been replaced adequately yet at 6, and their bowling attack bar Zaheer is a complete joke.