Bookmark+ is a great example of a feature (bookmarking things in Emacs) that seems simple, but admits a huge amount of thought and subtlety. Once the fundamental abstraction is nailed down, a huge amount of functionality emerges: bookmarks of URLs working in the same way as bookmarks of files, buffers, and lisp functions. In some ways, it reminds me of abo-abo's Hydra, except instead of proceeding from the abstract (hydras are functions providing an interface from which yet more functions can be dispatched) to the concrete (here's your menu for spell checking), it starts with something concrete (bookmarks), defines a unifying abstraction around it, and then turns everything into that abstraction, much the way Unix treats everything as a file.