Speaking as an end-user, not a Microsoft Exchange 2010 administrator:
1) In the default theme (apparently unchangeable by the end user), highlighted messages are displayed in a way-too-subtle shade of blue/grey. Unless you are looking at your LCD display exactly front-on, the highlighting tends to disappear. Result? Needless straining of peepers to see exactly what message you are looking at in a long list.
2) The end-user can apparently only switch the reading pane to the right-hand side (not the bottom) of the display. Thanks Microsoft, for assuming everyone using your products has access to a wide-screen display.
3) The sign-out button is nestled way-too close to the “Open Other Mailbox” drop-down arrow at the top-right of the interface. Result? Needless button-clicking to close a window inadvertently opened when attempting to log out.
Oh, and while it’s nice to finally have decent support for Firefox (gee, I guess all that marketing about premium browsers was a load of hooey), it also sucks that it’s limited to Linux, Mac and Windows platforms. Seriously, the little-known Sun Convergence email web app ran rings around this, and was totally cross platform to boot – although I’m sure Oracle are busy at work making the Java System Messaging Server prohibitively expensive as I type 🙂