From martinfowler.com:
There are numerous changes to the UML in UML 2 - it represents the biggest overhaul to the UML since the UML was originally agreed on. For general users the most obvious changes are probably:
UmlTwoDiagrams? are listed here: http://www.agilemodeling.com/essays/umlDiagrams.htm
Implementation of UML2.0 is incomplete. whilst the diagrams and modeling elements were adopted in October 2004, three other components (infrastructure, object constraint language, diagram interchange) are still in progress. Therefore the official UML version is still at 1.5.
ObjectManagementGroup now owns the BusinessProcessModelingNotation (BPMN), a frontend for BusinessProcessExecutionLanguage, as a result there are attmpts to merge UmlTwo with BPMN.
My take is that it mean UmlTwo will probably not see its acceptance in the user community for a few more years.
Activity diagrams are no longer ...
One of the enhancements in Activity diagrams, from the BusinessProcessManagement perspective, is the support of work flows. See a paper at http://csdl2.computer.org/comp/proceedings/hicss/2002/1435/09/14350291.pdf which address the use of Activity Diagrams for Workflow modeling.
UmlTwo Activity diagrams is said to have PetriNets forming the basis of its semantics, with its skin still looking very similar to UML 1.x activity diagram.
See also CategoryUml, UnifiedModelingLanguage