Turn-by-turn audible navigation, like RWGPS! I use a very basic computer without mapping capabilities, and like having the voice giving me directions from my back pocket - it's why I'm currently paying with RWGPS. If Strava's mapping capabilities were as good and they added that feature, I'd switch.
To be perfectly honest, I'd pay a little bit for past effort comparison. I could really care less about leaderboards - my days of waiting for a tailwind and KOM hunting are long past. It's just not worth $5/month, particularly with current job and salary uncertainties.