Power meter. Wheels I really like. Di2. Bike computer. (Watch in my case.) Handlebars that didn't come with the bike because I like these more. Carbon bottle cages because I wouldn't want to desecrate a nice frame with alloy ones.

Garmin's radar is a luxury I guess in the sense of not being common, but once you've used it a bit it feels like a necessity. I'm not sure if that makes the list or not, so there's its honorable mention.