Maybe it's just my poor imagination but how many plugins are truly unique to WP that you can't find on other CMSs? The only ones that come to mind would be those plugins that help connect to various B2B or B2C workflows, is that where the gold is mostly found?

WP essentially lets plugins do anything they want. The plugins are just scripts that register callbacks to events. WP calls events on BASICALLY EVERY FUNCTION. This is without exaggeration. I don't remember the exact names right now, but if you have a function like wp_get_title that gets the title of a post, there will be a "get_title" event that can modify which title is returned. So for every function first the data is computed using the default WP way, then plugins are allowed to discard all that work and replace it without something else entirely. There are events for deciding the canonical URL, for deciding the description of a post, for deciding whether RSS links will be displayed or not (the callback just returns true or false), etc.

In other words, every property can be modified through global event callbacks. Some events are called very early in the whole pipeline that let plugins just render whatever they want (e.g. render custom XML sitemaps).