I notice that WP.com doesn't tell us much about who comes to our blogs, so this might be a pointless suggestion. It's entirely possible they aren't keeping track. But if they are, or at least store IP addresses, then it would be very useful to know how much of my traffic is made up of return visitors.
For instance, a stat for "Return Visits (By IP)" which was a percentage, that would be very useful. Encouraging, too. I hate the feeling that the blog I have with heavy traffic is made up of new people every day. It would be nice to know how many people are coming to the blog regularly, as opposed to coming by once and leaving.
I do get the feeling that WP.com wants to encourage productivity in its bloggers, and this works both ways. It encourages someone with few return visitors to rework their content, and it encourages someone with a lot of return visitors to keep doing what they're doing.
Hope that puts it well enough. It's become almost a superstition with me not to come back and see the comments on any suggestions I leave. :-)