I’m a writer and I’ve got a pretty big vocabulary, if I do say so myself, but I do find myself searching for the right word sometimes. I like using Mac OS X’s built-in dictionary and thesaurus for that. It’s handy, especially in 10.11 El Capitan. Just highlight a word, right-click (yes, you can do that on Mac OS X, you’ve been able to for years) and choose “Look up [word]”, and you get a nice popup that includes synonyms.
The feature has been around since at least 2006. But as handy as it is, it seems like a no-brainer to me to be able to double-click on one of the suggestions and have it replace the highlighted word. Despite me trying this almost every time, it doesn’t actually work that way. So I’m filing Radar #25533454 for it. Just for good measure, I also sent Apple a more generic Feature Request via their Feedback page.