So @roleandp made an interesting tool - http://steemvp.com/ - that shows who is your "most loyal" follower, upvoting you most, and who is your "most valuable player", providing your with the greatest economic return. It's a really neat tool that is really enlightening!

As a new user, I kept getting messages that I didn't have enough Steem or SP to upvote more than once for every few comments that I left on different posts. I've seen different information about how much we should upvote (20 times to maximize our impact vs. 48 times in a 24-hour period). If we have more than a few folks in our feed and we are checking out more, we just can't upvote every blogpost, let alone every comment. I think a reply is what we can do. The Steemit help, on how posting rewards work page 6, says that replies reward the parent poster up to 6 levels deep, so there's incentive for sustained discussion. But I have no idea how to track that.

For me, the limitations in upvoting will be variety, quantity, and internet frequency. We each have to come up with the metrics that are meaningful to us.