IMHO the problem with D-Bus was that it was never small and utilitarian. They decided (correctly) to ignore all the engineering effort involved in performance and scalability, and put all that overengineering into the API instead.

D-Bus code is basically unreadable, as not only are the bus names heavily scoped (java-style) to avoid collisions, but also the interface and method names. A tiny python (or whatever) script to invoke a single method on a well-known object should be a one-liner but in practice lives over 6-7 lines just due to verbosity.

D-Bus types are inexplicably low-level for a "utilitarian" IPC mechanism, leading to a bunch of type conversion to do simple things, and a ton of marshalling code in the core. Javascript has shown us how far you can get with just IEEE doubles and UTF-8 strings, yet D-Bus suffers with a type model that looks more like C.