It would be really helpful to be able to see:

What percentage of due tasks had been completed on any day
How much time (according to the estimates) had been spent (overall, and on particular tags or lists).
And perhaps, other time-management evaluation information...

Such functionality would also require an "I did this on a former day" operation to keep it accurate.

It could be added on another tab after "completed" so it didn't get in the way of any user who didn't like it.

I find RTM a really good to-do list, but I find it really frustrating that it gives me no picture of how well I'm doing or what might need to be changed in my planning in future.

RTM would be totally amazing if it helped me keep in touch with the reality of what actually got done, and it would be really encouraging to be able to see more about how much I have achieved in addition to how much there is left to do ;-)