Why do Archicad and Revit only share 30 or 40% of the global architectural sofware market ? Maybe because lots of small practices find these software unadapted to the way they work, and to their bank account.
I think there's place for an open software that doesn't work like the existing. It will be simple in its tools and processes, and will have infinite capabilities.
Usefull extensions for Revit Architecture 2008, AutoCAD and all vertical products (ADT, Civil 3D, ...) for versions: 2006, 2007 and 2008. Read documentation (find it in download section) for more info.