How much game do you need to support a monthly fee?

I'm still happily playing the Football Manager Live beta, but I'm painfully aware that it is very much a niche game. On my beta server there is a large number of players inactive, for example in the league I'm in 13 out of 19 players haven't been online in the last 10 days. When FML gets released, there will be a core of very enthusiastic fans, but the average computer gamer will just look at the game shortly and then give up, because the complexity is high, and the graphics are low. In the particular business model of FML that means he downloads the game for free, doesn't get a free trial, so he pays for 1 month of fee, and gives up before the end of the month. So I was wondering whether a different business model wouldn't have been more appropriate, like a higher cost of buying the game, no monthly fees for a basic version in which the stats are condensed into easier to understand aggregate values, and a monthly fee for a luxury version for the hardcore fans who want all the details.

That got me wondering for what people are willing to pay a monthly fee, and for what they wouldn't. Playing Diablo 1 or 2 on Battle.net has no monthly fee, and if Blizzard isn't changing their business model, Diablo 3 will still be playable online in multiplayer mode with all your friends for free. Is it really such a huge difference between World of Warcraft and Diablo multiplayer that the one would require a monthly fee and the other wouldn't support one? And if WoW still supports a monthly fee, what about the less popular MMOs? Would Hellgate London been better off without its partial monthly fee subscription model for fans? Would Tabula Rasa have sold better if it had been free-to-play with microtransactions? Would Guild Wars have sold 5 million copies if it had a monthly fee?

I don't think the monthly fee subscription model is ideal for every game. There are some games I just wouldn't pay a monthly fee for, even if they are nominally MMORPGs. Although I'm not so poor that I couldn't afford it, I refused to pay 2 Euro more per month for Age of Conan than World of Warcraft costs (WoW is 12.99€ per month including taxes, AoC is 12.99€ excluding taxes, 14.94€ including taxes, for month-by-month subscription in both cases.). But I donated $20 voluntarily for Kingdom of Loathing, and not just because that gives in-game items. And then there are certainly games I wouldn't pay for, but other people would, or vice versa, so the rules on how much game you need to support a monthly fee aren't universal.

So what do you think? What are you willing to pay a monthly fee for, and where do you balk at the cost? Are there specific features which you think justify a monthly fee?