The support libraries add a specific subset of features, they do not aim to enable backwards compatibility for every feature added since 2.3.
So check out the list of compatibility features here: http://developer.android.com/tools/support-library/features.html
And compare it against the list of new platform features: http://developer.android.com/about/versions/android-4.0.html
To figure out what you're missing out on.
Edit: one quick example which comes to mind is the NumberPicker controller, which is in 3.0+ but NOT included in the support libraries (at time of writing)