Don't Try to Reminplement HTML in your Rectangle of Joy (redux)
As a brief addendum to my previous post about HTML, I just thought I'd point out that Google's Webmaster Tools work on my phone while Bing's do not.
Obviously, checking on your website from your phone may not be a core use case (after all, the only reason I know it works on my phone is that I've been travelling and just happened to try it out while away from my "real" computer) but phones and other mobile devices are an increasingly important way people use web applications. Consequently, if your web app lives inside a Flash or Silverlight rectangle you're cutting off an entire class of uses.