I think a really good feature, if possible to implement, is to give concurrency stats, rather than just overall player numbers. Even if a server has 3x as many alliance characters as horde, since the servers have population caps, you may have 2000 alliance, and 2000 horde, regardless of the fact that this may be 100% of the horde's active population and 33% of the alliance's active population.
To be more accurate for the sake of things like ganking probabilities, 1.12 world pvp equality, and battleground queues, it would be nice to know what the average concurency rate is.
Perhaps this could be implemented by somehow taking timestamps with the census, and matching censes taken close enough together? I don't know how often this sort of thing happens though, just a suggestion!
Concurrent Stats
There is player collected concurrency numbers available for each server if you delve the server data, how often the data is collected is entirely up to the people contributing on that specific server. The problem of course is that more than a few realms simply have too few samples for the data to be good, that essentially messes up any overall stats for server clusters and all servers as well.
