Huh? ArcGIS is wholly extendable, and complete businesses are based on the Arc extension ecosystem, using COM. AutoCAD can (could?) be extended using COM. I've personally written extendable systems using COM. COM is a hugely successful component interface standard. Name any other standard with the same speed, flexibility and adoption.
Again, I'll agree that it's not trivial to implement a COM object. You need good understanding of C(++), the Windows API, threading, and some more things. There are tools to make it easier (e.g. to write a COM object in Visual Basic) but they generally don't make for robust components. In the hands of a competent, experienced developer, COM is a massively powerful tool.
COM is: "Unlike C++, COM provides a stable ABI that does not change between compiler releases. This makes COM interfaces attractive for object-oriented C++ libraries that are to be used by clients compiled using different compiler versions." http://en.wikipedia.org/wiki/Component_Object_Model
Others are talking about the compound document format OLE. And about DCOM (Distributed Component Object Model). Both OLE 1+2 and DCOM are not that great implementation wise, in retrospect. COM on the otherside is fine.
D-Bus is like DCOM, and has similar/other deficits, as do the competition: CORBA, RMI, XML-RPC, SOAP.
MDI and compound document format were an oversold concept, that failed. Today we have HTML5 with iframes.
Again, I'll agree that it's not trivial to implement a COM object. You need good understanding of C(++), the Windows API, threading, and some more things. There are tools to make it easier (e.g. to write a COM object in Visual Basic) but they generally don't make for robust components. In the hands of a competent, experienced developer, COM is a massively powerful tool.