Stay clear of class names that are PHP keywords. Example: "F3_Package_Controller_New" becomes "New" when adding namespaces, "F3_Blah_View_Empty" becomes "Empty", ... Rule: all reserved keywords are disallowed. But using "EmptyView" or "DefaultController" is cleaner style anyway!
Watch out for that global namespace! Type hints like "Exception $e" will break - the current namespace is applied first, no fallback is applied for classs or interfaces! Use "\Exception $e" in cases like that. This includes instantiations of PHP internal classes as well, e.g. DateTime, ArrayObject, ... The same is true for any other non-namespaced classes you use in your code (look at the Lucene or PHPUnit parts of the code).
Always use the fully qualified name (for now). I.e. even in a class always type the full name, as before, just with \ instead of _. This avoids problems with ambiguity, class loading, name resolution and changing the world climate until we have a better understanding of a) best practices on using namespaces b) how to refactor things if names are unique only if you know about the namespace of their scope
DEV3 now complains about syntax errors in TypoScript2 files as it does not yet know about the new syntax for component names.