Well here is what I think of the current payment methods:
P2P (WoW): The pay every month method I feel is solid because it helps for server costs and helps the devs grow the game even if the playerbase doesn't. But I seriously hate the fact that you are pressed to play just that game and grind in it for a month, just because your forked over your money for it. Still, easily the best method because it doesn't feel like you are being ripped off, and you are getting a growing experience as you pay, which more than makes up for the cost.
P2P with purchasable time in game (EvE online): This is probably my second favorite. This method allows players who really enjoy the game to play the game for free (assuming they can manipulate the market and make enough in game money) and those who still want to play to just do the monthly fee. However, this does come with the problem where you feel pressed to play enough so you don't have to pay. Of course, not a problem if the game is actually decent.
P2P/F2P with micro-transactions (LoTRO, LoL): Yuck. How could you hate your players so much that you feel the need to charge them every month, maybe put some expansions in, AND make the customers pay small amounts of money again for items? You would have to be crazy money hoarders for this one. Not so bad if those micro-transactions are purely cosmetic, since players can buy those at their own will, but still a bit of a money grub. I ESPECIALLY hate the games where real money means power, how is throwing money at your screen fun?
On the other hand, I feel that F2P with cosmetic micro-transactions works REALLY well for many reasons. The artists/sound/whatever team can work on the design for the cosmetic item, while the rest of the team gets ready for the next update or something. I don't see a problem with cutting the grind, or looking awesome, but I do have a problem when money means power.
F2P with subscription (Runescape): I really don't have a large problem with these, but they lack any sort of competition, due to the fact that subscribers more often than not get a huge advantage over free players. At least in something like RS, free players are seperate to members and if a member logs on to a F2P world, they have to use F2P items and areas. Definitely better options though
That is pretty much all of them. If none of the above were mentioned and are used, you are most likely doing it wrong. The only one I didn't mention was the pay once method, which is by far the greatest method, but really doesn't work for MMOs unless the creator is getting income from somewhere else. Also, I don't know any MMOs that use this method apart from GW2, which has not come out (did GW1 use this?). I'm not too sure whether this method is the way of the future, as it just isn't going to handle server costs, not to mention extra content that players will no doubt expect from an MMO.