The currently available SL3 build is just a port of the SL2 controls, so there are no new features.
For the SP1 release that is scheduled for next week we will support the data validation framework. Our controls will be modified in order to use the data validation and to integrate the validation UI.
We are also fixing the Out of browser issues that we have - now our controls will work in the OOB scenarios.
We are also adding support for RIA services. You will be able to use the validation and the data related controls that come with the RIA services in order to page/filter and sort the data bound controls.
There are ofcourse will be some breaking changes that are due to the SL2-> SL3 conversion, but they are mostly minor and will be documented with the SP1 release.
Please let us know if you need more info.
All the best,
the Telerik team