I would love to see which people have the most network-wide reputation, how can I see that?
Although a feature like this sounds very cool and could be implemented without much effort (I think), I do wonder what will be the usefulness of it.
Reputation among sites isn't exchangeable. The ratio on answers and up-votes differ a lot among sites. Also on some sites people are more willing to vote than others.
We do not want people to 'bet' on that other site because it has a higher ratio than the lower traffic, less popular site, which could use some hands.
Some figures, going on Martijn Pieters' comment:
Take the current years reputation (so from Jan 2015 until Aug 2015) of the highest scoring users on Stack Overflow and Mathematics (second most active site in the network): SO 70K, Mathematics 46K. And the top user in Seasoned Advice has an astonishing 7K! There is no way for a user in Seasoned Advice or even in Mathematics to come close to the SO user. No matter how much they try.
I think that the side effects of this feature must be thought of first. Then we should see the nice feature.