Neowin member, blogger and Microsoft Connect tester "tuxplorer" has posted an extensive list of features that have been removed from the transition from IE8 to 9 on his blog, and despite reporting these issues to Microsoft, he felt that his concerns weren't addressed.
IE9 has received praise...
browser
comparisons
customization
download manager
features
history management
ie9
internet explorer
javascript
notificationbar
privacy
rendering engine
settings
surveillance
tabs
tech news
trident
user experience
user interface
web standards