I would like a clock (as per hack somewhere else on these forums) as standard AND estimated number of pages left in battery!
I know that may sound daft but with a device that does not allow you to carry a spare battery around this has been the only pain for me. 3 times now I have got on the train only to find after a couple of pages it turns off since the battery is too low.
Now you may say - just regularly charge it up! But, as we all know by now with mobile batteries being the worst culprits, these lithium-ion have the longest life if you charge fully & then discharge fully before recharging again. When you have no ability to (easily) swap out the battery this becomes even more important (even if it means I take something else to read with me on the occasion it does run out on the road, erm track

)