I feel at this point in time it really needs to be stated that building plugins that extend the functionality of WordPress is just as important for the ecosystem — possibly more, depending on the nature of the plugin — as contributing to core.
WordPress isn’t as huge as it is because of blogs. It’s huge because plugins make it viable as a general purpose CMS. In fact, there’s one particular plugin that is more responsible for its general-purpose flexibility than any other.
If WordPress were limited to its core functionality, no one would even be talking about it today… because it would have been abandoned at least a decade ago.
That is all.