On April 21-23, 2008 I will be in Washington DC at Architecture and Process conference to present a panel on XPDL weakenesses.
Here are the main ones I found starting from the analysis of the example presented on XPDL specification document [WFMC-TC-1025]:

• an high complexity of the diagram
• a great fragmentation of the information
• a weak referential integrity (there are no unique IDs inside the XML-Schema definition)
• very complex queries
  

On BPeX project page it is possible to download a more detailed description of these points.

Fig. 1: EOrder main process

Fig. 2: The XPDL version

Fig. 3: The BPeX version

-- mchinosi