So far I've encountered a couple issues, one which truly boggles the mind.

The first one is the one you're most likely to encounter after upgrading

 Fatal error: Call to a member function toHtml() on a non-object in <somewhere>/app/code/core/Mage/Core/Model/Layout.php on line 52

The solution is explained here:

Basically edit your theme's layout/page.xml and change <block type="core/profiler" output="toHtml"/> to <block type="core/profiler" output="toHtml" name="core_profiler"/>

The next two are frankly amazing in that they ever made it into the release.

See here:

To quote the finder of these issues 'Yes, preparing a shipment prepares an invoice instead..'