Dimitris has an interesting point about using Ajax in your apps. Using it in a few carefully selected places can really improve the experience. HelpSpot uses Ajax in 5-6 places I think and there's some more coming in the next release. My current favorite is a little flag that lets you mark a request as being unread, providing yourself a reminder to go back and check on it. Before Ajax, this would have been a page reload and really annoying. So annoying that I'm sure I wouldn't have even added the feature.

On the other end you really do need to be careful. I've been prototyping the addition of Ajax to the request form, allowing full submission via Ajax along with queue switching via Ajax. These should be better with it, but in practice they aren't or at least in my current prototype they aren't. They load in ways you don't expect that are jarring. Also, you run into URL issues and being able to easily link or open up new tabs with separate URL's and so on. It really is playing with fire.