A lot of folk tend to forget that we're talking about elite sportsmen - if you have an off day as an individual or as a team (as England did in days 1-4 of the last test) you will get found out.
Unfortunately, England aren't as good as some people -and possibly they - think. The Ashes will be close, though a lot will depend on Clarke's fitness. Going into an Ashes having lost Ponting, Hussey and possibly Clarke would be huge. England currently have the experience that the Aussies had for years in Hayden, Langer, Ponting, Gilchrist, Warne, McGrath, etc - but look what happened in 2005...