The last three clients I was brought in to help all had what could be called Web 2.0 centric applications.  I was not privy to the underlying architecture and was not involved in the design process so am not totally sure about their overall Web 2.0'ishness.  Two of them employ Flex and the other one, Ajax.  The first thing I encountered was the increase in difficulty in creating a meaningful load-test script, in order to do that we had to use two great utilities, ServiceCapture and Fiddler.  Instead of being able to capture all necessary variables we had to painstakingly build them by hand and one call had 45 form variables.  

The next thing I am observing is the increase in the amount of work at the browser end.  This concerns me because we typically can control our server side environments, we know what systems we have there, the network etc.  However, we have no control over what is at the client end and flakiness there can have various consequences.  I have felt for sometime that Flex is better for Web 2.0 type functionality than Ajax, I realize they can both have their place though.  At least with Flex we are relying on a somewhat predictable environment with the Flash Player VM.  Also, I wonder if the possible in-browser issues might make AIR even more compelling.

Does anyone have any thoughts or observations on this issue?