Simply put - the full history of EVERYTHING I ever listened to.
Not last 50 songs, not recent artists\song played, just a simple list of every track I *EVER* played and the date/time I played it.
So many were the times I listened to a track I didn't save, and I could not find it again! Furthermore, this can let people see what they listened to in different episodes of their life and cry/laugh/be shocked. Sure, you can add stats and filters and graphs and all, but lets start with a simple, searchable list... The data is redundant in size, I don't see a reason why NOT to include this feature.