Tim Sneath says:
One of the greatest drawbacks of AJAX is that it's simply too hard to code. The very fact that I can count the number of truly exploitative public AJAX applications on the fingers of one hand is an indication of that. [via Tim Sneath]
What is still missing (from MS) is a really good IDE + Debugging model (it is implied part of Atlas' job is to address this) - I still use a text editor to write fairly complex apps and I am not a rocket scientist by any means, not a particularly talented programmer and not obsessed by the niceties of all your tags in lower case. But, I do use Zeepe which provides a great framework for doing a lot of standard UI gunk, saving the huge amounts of code that would otherwise lead to massive amounts of code.