This article on InfoWorld is worth a read.  Probably nothing that anyone didn't know already if you have spent any time working intensively with javascript in the browser.  I would add the the subtle differences between browsers is another big thing to add to the list of challenges in this environment.  I would disagree somewhat with the statement that people inherently mistrust proprietary alternatives like Flash or Silverlight.  End users certainly don't care and based on the adoption of Flash and momentum around Silverlight, a very large percentage of developers don't either. Yes, there are a vocal group that care a lot, but the vast majority of developers just want to get the job done, not troll through source.