For a while, I've been saying that grinding in an MMO is ok if it's just for looks. Don't make people grind for power, but if they want some spiffy looking armor, they have to log up some hours.
But screw that, I have an even better idea, which occurred to me over long hours of brain inactivity while grinding in CoX recently thanks to the wonderful ission Architect XP bugs.
If someone's giving you $15/month, I'd probably say you should just reward them with some fluff, whether they've killed 1,000 foozles or not, whether they even log in or not. Then, if they want to bash their head against your gameplay repeatedly because it's actually fun, then they can feel free to (and if they actually do without you hanging the industry standard carrot over their heads, congratulations, you are not going to designer hell).
This is actually similar to EVE's system, in that you accrue stuff with real-world time spent, not game-time. Of course the difference here is that you don't make the older players more powerful... you just make them look awesome. The most flashy stuff in the game can take a few months to get, that way your veterans can still distinguish themselves, and new players are lured to keep subscribing.... but nobody has to grind. Unless they really want to.
This can only be win-win to me... thoughts?