Ajax and usability.
We all know the drill, fill in the boxes and click the button, the page posts up to the server, we watch the progress bar and we wait for 4mb of viewstate to come back down, ticking away to it's self.
Ajax.
What's my page doing now? I've clicked the button ... there's no status bar, no progress showing, nothing to let me know where my request is at (has it finished posting? am I waiting for the response? has it crashed? did the button click even get picked up?).
So, what's needed to fix this?
Well, for start, a progress bar that people could use would be a great help. Flash designers and developers have used these for years. Please, Ajax developer, use progress bars - not just some animated gif which loops, because that would be irritating.

Or how about this ...
We've filled a long form in, pressed the button and we're now waiting for the response, except, wait, what's this? I've spotted a mistake! I'll press "Stop", that stops things for me ... or does it? Not in Ajax.
Back, Refresh, Go ... all are gone, users normal methods of doing things are broken.
Seriously guys, you need to think about existing customers and the millions of web users that know how to use current applications when designing new applications.
One thing I'd really say is a bad idea is mixing Ajax code with no Ajax code on the same page.
What do you guys think?