It was a few different things, and it only lasted for the first month or so of using Anki. During that time I occasionally had moments in conversation when I grasped for one of the words in my deck, when normally I would smoothly talk around the idea with simpler words. Sometimes I succeeded in incorporating a new word into my speech, but the usage was awkward. Sometimes my interlocutor didn't know what the new word meant, and not only did I have to explain in simpler terms, I came off as out of touch and a bit of a know-it-all. It was a little uncomfortable at the time, but the harmful effects did fade as I became more aware of which words were ok to say in what contexts.

The primary benefit has been a better understanding of the written word rather than a larger productive vocabulary.