Thanks for this fascinating article! But my problem is not one of motivation, but one of knowledge. i.e.: I have no idea what the most efficient use of my time is. I just do what I feel like doing, not (necessarily) what gives the best R.O.I.

If the analysis is time consuming/paralyzing, take a guess and proceed. If you literally have no clue, chat with your co-founders. If you're still unsure, grab 10 random target users and give them the list-- as them which thing on the list they'd want you to work on.

Perhaps you could use the Adwords test as described in number crunchers: create Google Ads for the various features you could be working on, and see which ones get clicked more. The ads could point to messages of the form "we are working on feature X".