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:

* Interaction Frame notation added to sequence diagrams
* Collaboration diagrams renamed to communication diagrams
* Activity diagrams are no longer a special case of state diagrams - as a result they get a host of new things
* A notation for composite structures allows you to describe classes hierarchically

''Am I the only one who sees the irony in Unified Modeling Language. . . version 2?  I'll stick with the GalacticModelingLanguage thank you. It doesn't need to change versions.''  I take your point... especially when the count of "unified" diagrams has increased from 9 to 13.  At least the '''authors''' have unified... now can we have some factoring please?

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.

* It is difficult to understand how come there is no news of progress with the above missing elements of UmlTwo for years, and then OMG (in July 2006) tout the adoption of System Modeling Language (SysML) based on UmlTwo. Note the SysML development is cosponsored by International Council on Systems Engineering INCOSE) and uses a subset of UML2, and have elements of the "missing links" in UmlTwo. See announcement at http://home.businesswire.com/portal/site/google/index.jsp?ndmViewId=news_view&newsId=20060706005733&newsLang=en 

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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.

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''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.

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See also CategoryUml, UnifiedModelingLanguage