I like the idea of weighting the problems by a difficulty score that is calculated from the number of users solving it. I think the idea as presented ("A perso

I like the idea of weighting the problems by a difficulty score that is calculated from the number of users solving it. I think the idea as presented ("A person's score is the sum of the difficulty ranking of every problem they've solved") would not work great on its own because there will be a large number of people who have solved every single problem. But certainly we could do the thing about adding up people's per-problem ranking with a weighting based on the problem difficulty! I will experiment with this. The thing I don't like is that it can result in the leaderboard changing, even for people who have solved all problems, simply because more new people attempt a particular problem, which can alter the relative weightings: Imagine Alice has 1st place on problem 1 and 2nd place on problem 2. Bob has 2nd place on problem 1 and 1st place on problem 2. Problem 1 is considered twice as difficult as problem 2, so Alice is ahead of Bob overall. Bob doesn't like this. He wants to be ahead of Alice. He signs up for thousands of accounts to attempt (but not solve) problem 2. This makes problem 2 look very difficult, so now problem 2 is worth more than problem 1 and Bob is ranked ahead of Alice.