Not all model compilers desired (even those desired by Mellor) are built yet. (Executable UML book, chapter 1.4 Model Compilers)
No standard syntax defined for object actions yet.
Still an area of research, not established industry standard. It would be great to see some balanced commentary in this article - My feeling is that the core reason the write-up here is pitching xtUML as the shangri-la for software development is that for the most part xUML currently applies best in academic rather than real-world industrial settings. And what approach doesn't look like the answer-to-everything when it's still more talk than walk!)
Copyedit (should be OK, but needs someone other than me to look over it ;-)
Wikify
links to other useful articles (still to go: syntax)
