It's hard to find an article on MMOs that doesn't include metrics of some stripe: registered users, peak concurrent users, et cetera et cetera ad nauseam. With the expansion of free-to-play and ad supported games, it's not as easy to compare games as it was when everyone operated on a subscription basis; but Ron Williams of CDC Games (the Chinese company that has a stable of wildly popular games in Asia and is expanding into the West with Lunia). While this set of metrics may not have any impact on the average user (as long as you like the game, what difference does it make if a bunch of acronyms are being converted into other acronyms - or not), but they do provide a good base for companies figuring out how to tweak their offerings:
An online game is just online content that you need to market in order to sell. The number of potential customers you can drive to the game's website either through word of mouth or through marketing spend is the key driver of sales, just like any other online business. The UV trend line is the best indicator of sales potential of a game.
Considering the amount of time I spend plowing through press releases for new MMOs and releases bragging about the earning potential of those currently on the market, it's interesting to get inside perspective on what all those pesky statistics actually mean from an operator's perspective.
How To Compare Online Gaming Businesses [Gamasutra]