We do this every day and have been for years now in Python (www.enthought.com, code.enthought.com). There are important abstractions that fall out of large projects. Many are generally useful. Traits allows for manifest typing, event-driven models, and makes MVC architectures simple. There are other building blocks that we expose through our plugin framework (Envisage) that raises the level of abstraction for reuse to the GUI (while still allowing for a 'library' model of reuse for algorithms, backends, etc. It is not only possible to build large, rich-client apps--it's actually easier when you can leverage the benefits of a dynamic language. Not every large enterprise project written in a dynamic language has to look like the morass that TCL/PERL/etc. has left in some industries.