It's a good question. As a gamer and MMO enthusiast, I've played subscription games, buy to play and free to play and each model has its merits and flaws. The free to plays have no price tag, but tend to be games that were either poor quality to begin with or didn't meet their creator's expectations, so now they're keeping it around just to make some extra cash from microtransactions. Buy to plays tend to have professional-grade content far more often, but run the risk of still locking out a lot of content if you don't buy the features the designers deem unnecessary for the game's core experience and instead put in the cash shop. Subscription games provide plenty of top-grade content to all its players, thereby avoiding the risk of getting labeled "Pay to Win" or "Microtransactions: The Game", but they can come across as exorbitant if they give you a jacked up rate, especially if you don't play it every day.
To me, it hinges on three things: subscription price, content and scheduling. I know that's pretty broad, so I'll try to clarify. For starters, I know that WoW started the tradition of $15 per month, but who says that's where a subscription HAS to be? If games like GW2 can operate without a subscription fee at all, maintaining their servers and providing regular content updates with game-sales and cash shop purchases alone, then why couldn't another game have a hybrid system where there's a subscription fee, but a much smaller one that's supplemented with a cash shop? Imagine a game that has a monthly fee of $10 or even $5 per month that also offers an in-game cash shop with some nice luxury items, like special armor and weapon skins, mounts, emotes, etc. I don't see why the $15 per month rate has to be treated like its sacred.
Content, naturally, means the quality of what I'm getting for my money. I like a game with lots of in-game features, particularly roleplaying features. Customizable appearances, player housing, interactive objects, emotes, these things add character and appeal to a game for me. An overarching story for the background is definitely appealing as well, though one thing I've noticed about the "theme park" model of MMO is I tend to feel like the plot is trying to simultaneously make me the star while also threatening to leave me behind if I don't keep up (I'll get more into this when I talk about scheduling). Because of that, I enjoy a good backstory that I can explore at my own pace, or the opportunity to develop my own story with others through roleplay.
Finally, there's scheduling. Sometimes, after playing one particular game for a while, I feel like taking a break. Or I feel like playing a different game for a while. I don't exclusively play one game at a time (though maybe I should if I want to get through any of them...) so I don't require an MMO that has new content coming out every week. I've said this before in other threads, so I'll try to quickly summarize: an MMO that comes out with content patches and plot changes too fast makes me feel rushed and sucks the fun out of the game for me because I'm forced to grind content rather than relax and savor it. Someone in this thread mentioned monthly content updates; I could get behind that. It certainly sounds like a better pace than the every-two-weeks schedule GW2 seems to have adopted (which, incidentally, was what sadly turned me off that game). The other option is to let the players create their own stories rather than have the developers try to play dungeon master. I put this idea out there in a different thread, so I'll not rehash it too much. Suffice to say I know this model could be risky, but it would likely be very well received by the dedicated roleplayers who like to decide their own path in a game.
So that's the stuff I'd want to see in anything I'd pay a subscription for; reasonable (i.e. lower than $15 a month) pricing, versatile gameplay features and an update schedule that gives me time to deal with what's on my plate already. I don't know if I'll ever find a game that has all that, but hell, a guy can hope.
