I'm trying to compile a list of wants and don't wants for a multiplayer RPG, and would love ideas and comments.
Runescape's business model is probably my largest inspiration, although I'm not too impressed with their premium tokens.

* = Must consider what happens when premium status ends
**=Challenging with a mixed population server

Want:

Free play without level restrictions with access to all skills
Optional monthly premium membership subscription (€4-7)
Extra stash *
Premium only areas (** See premium only items)

Leaning towards want:
Premium only items (obtainable in premium only areas, not usable/obtainable for non-premium members) **, *
Player housing option * (costs non-premium currency or ressources, only possible for premium members)
Respec option (costs non-premium currency or ressources, only possible for premium members)

Leaning towards don't want: (Grey area pay-to-win)
Ad removal (this is a desktop game - not sure if ads are even feasible,
and I want the player to keep the focus on the game and/or my site)
Grind reduction: Experience, luck, gold boost
Premium currency for vanity items
Premium vanity items* (Does anyone actually want that? Path of Exile's survival seems to suggest this though...)
Respecs for premium currency

Don't want: (pay-to-win)
Ingame currency, items, skills, direct PVP/PVM advantages in return for subscription
Premium currency for anything other than cosmetics
Tradeable premium currency
Tradeable premium status tokens

Pay for Cosmetic options do seem to be a pretty decent path to look at. While a lot of players aren't interested in them, many actually are.

Step back and look at what you want the core gameplay to be, and then look at what kind of optional stuff that doesn't have any direct impact on that core sector could exist.

Also consider looking at it as a totally free to play game, all players are your 'customers', but the paying ones become your patrons. What kind of things can your patrons get that a regular common player might not be entitled to? Well, how about beta access? Test server access? Elite forum access with a more direct interaction with the developers?

Is your game possibly able to run in layers of functionality? All players could be soldiers and fighters, and everyone gets the same kind of access to the core levels that give the same kind of loot and fight the same kind of mobs and bosses as everyone else does, but is a regular player in anyway negatively impacted by the patrons all running off for special side quests or GM led special events? Maybe give the base level free to play types enough content to keep them interested in the game, but offer just enough 'extra stuff' to tempt them into coughing up cash in return for something a little different (and keeping the servers running). It doesn't make them any more powerful when the patrons come back into the 'main game' sections with everyone else, they'll just look a little flashier, and have seen more things than the other players.

Other options could be, all players get to be soldiers and fighters, but who gets to own shops, or guilds, or maybe even entire towns? A well designed and tiered system could see a fair number of players choosing to fork over a bit of cash, a few forking over a chunk of cash, and a select few players forking over a ton of cash. Things like 'everyone can be part of a faction, but only X can join a guild. Only Y can own a guild, and only Z can sit on a faction "council"' or something along those lines. - Faction council member dies to the big bad boss in a dungeon just as readily as the non-paying faction member.

I was actually wondering if I should even post this thread thinking it was a bit of a dead horse having been discussed many times all over the net.
I'm glad I did - you people really took your time to give some very thought-provoking input - thanks!

@[member='Luckless'],
Yeah I'm not against pay for cosmetics ideologically - the only thing that rubs me the wrong way is that I want a quite gritty world without too many wings, halos etc.
Perhaps find a way to make people pay for more grit:)

I love your idea of only opening up certain play modes to premium members - clan leadership, shop ownership etc. I think though that I want both free and premium members to be able to join clans - but with a limited set of the advantages that come with being in a clan.
I also had some ideas about how you could customize forum avatars based on your characters - you just gave me an idea to allow more detailed ingame "picture taking" for premium members, while only showing a basic avatar image for free members.

@[member='Paragon123'],
It's not just that I don't want to show ads, but I'm not entirely sure ads in a fullscreen desktop game will work well. If you know of any ad networks that will accept ingame ads in desktop games I would be happy to hear about it though. And I don't want to advertise other games:)

I don't necessarily want everyone on a totally level playing field. Players need to have a reason to want to pay.
Giving premium members an advantage is not real pay-to-win for me, if the idea is that free membership is like an extended trial version, while premium membership is the way to play the game for real.
Giving players the opportunity to gain extra advantages by spending more cash is the kind of play-to-win I want nothing to do with whatsoever.

Having items that can only be found in premium only areas is something I most likely want.
If free members can obtain those items in a normal trade using ingame currency then it's not a problem for me.
Having those items be premium only and not usable by free members borders on pay-to-win.

Allowing players to trade premium currrency to other players is the equivalent of selling ingame currency or items directly to players for real money. This is definitely pay-to-win for me.

Actually premium currency for anything but vanity items - and perhaps for stash tabs - is not something I like very much because I only want 2 kinds of players: Free and premium. Ideally if someone decides they want the full experience of the game then they obtain premium membership and that's it, as long as the membership is maintained.

Losing ingame housing because premium membership is not renewed is not something I'm super happy about - perhaps I can restrict access or use until membership is paid again.

@[member='hypester'],
I like the idea of early access, the way it's done in Shroud of the Avatar where Kickstarter backers are given beta access is brilliant. Never played the game myself though :)
Back when Path of Exile was in closed beta I bought forum gold at D2JSP just to buy a beta invitation for Path of Exile - I would much rather have seen that money go to Grinding Gear Games, but that wasn't an option back then I think.
Love your Bartle reference :) Actually I'm definitely going to be rewarding achievers for premium membership because they gain access to premium areas. Not sure what I can do for killers - perhaps access to PVP leaderboards.

Do some market research.

Look at other games that have the model you want. There are many games, especially in the mobile space and MMO space, which have cosmetic only, or non-balance affecting freemium paid items.

Granting users access to premium only areas is a good idea already mentioned. You can also flag users who are paid members with a special badge, give them access to different areas on your games forums, give them higher priority technical support, and lots more.

The 'cosmetic' items you speak off appeal to 'whales', who if they like your game will spend money regardless of it if helps them win or not. In some ways, the freemium model appeals to addictive personalities and in some way mimics a casino. Some people go into a casino not for the desire to have more money and winnings, but for the attention they get when they win. These people already have lots of money.

Anything that can be used by the player to go "look at me! i spent money on my character! I have a purple hat and you don't" will likely sell a lot better than you expect.

Don't neglect marketing though. Simply putting the freemium items there and hoping people bite won't work. You need to advertise quite heavily, and at every opportunity make the players aware they can buy these things, without it being over the top "in your face" all the time.

Perhaps you can also award the paid items occaisionally for non-payment actions, e.g. "today only: share the game on twitter and get a free purple hat, usually $0.99". These help boost sales of freemium items indirectly, as when others see people with that hat after the one time offer has expired will want it, and they have two choices: wait for the next one time offer, or go out and buy it.

Hope this helps!

However an important thing to keep in mind when planning your 'promotional' offers of stuff: Be VERY careful of just what you've promised people who had PAID for something.

Plan things out, and keep extremely careful notes on what is said and under what conditions stuff is sold to players. If you forget that you initially told players that your snazzy "Purple Hat" was limited edition, and only available if you pay X and do Y, and a few months/years later you turn around and say "Free hats for EVERYONE!", then you're setting yourself up for pissing off a VERY vocal member of the community, who you will then have to deal with and put up with a PR nightmare.

Also remember to design things such that non-paying users get to glimpse what the paying users are getting. Occasional free weekends to let everyone come poke around in the "Premium area" and then slam the gates shut on them and kick them back out with the 'plebs' can convince players that "Yes, I Do want to pay this small and easily affordable price to get this all the time..."

Another big point to remember in freemium design: Don't nag the player, and don't hide ALL the fun behind paywalls. You need to maintain a very careful balance here between showing off why the player should want to pay you more than they currently are and letting them decide for themselves that they actually find the game fun in and of itself.

I very rarely play freemium games because they're usually horribly boring, and very poorly presented with regards to what you get out of what you pay, and make it hard to predict how much you need to keep paying to actually keep having fun.

If your game readily becomes boring to the player unless they pay more than they have, then you're setting yourself up for the player to decide to leave and find something better.

In short: Make the game Fun at each pay level, and make it stay fun for entire play sessions. Make paying more unlock new fun, not more of the same fun you were having till you hit the pay wall. If I am mid way through my play session and I'm hit with a paywall "Give me money, or wait for the timer to go down" kind of thing... Well, personally I'm FAR more likely to just go find something else to do while that timer runs down, and then it becomes a real toss up whether or not I bother coming back.