Long term statistics are very useful, because they allow you to keep your database size small while keeping relevant information for a long time. Currently long term statistics only work on numeric sensors, storing a min, max and avg value per hour.
I would like to see long term statistics on binary sensors, storing for how long a binary sensor was active in a given time frame. A window sensor could for example store a window-open-time and window-closed-time per hour. It would then be possible to add those up to calculate for example how long the window was open the last 4 weeks.
This could then also be extended to other stateful entities, such as persons, allowing me to check for example how much time I spent at the office in the last year.