When making a game what do you aim for, and why? What do the players want? What makes a game of mafia good?

One point is obvious; Balance
If a game is not at least somewhat fair then its boring to play, with one side quickly taking control of the game and getting an insurmountable lead. Having it very hard for a Wolf or other "Single Neutral who wants everyone dead" role is acceptable, its fun for the player and making a wolf with the power of a medium size mafia would be very hard. In games with multiple groups, especially if info is available reasonably easily, this tends to be less of a problem. The weaker groups have an interest in ganging up on whoever is top dog at the time, however if no one knows much about alliances outside their team one team can take charge and play the others against eachother.

Balencing a game requires some thought, but so long as you have a bit of experence with mafia its not too hard.

Another thing which helps make the game enjoyable to more people, and I feel has been often lost in larger games, is this: Make the players think for themselves
By that I don't mean "make a complex game with a load of crazy stuff so they have to work out what's going on", but "Don't set the game up so half the players best stratergy is to be a pawn for someone they trust", if there is a small group of players that have been "confirmed" trustworthy then for village players there really is no reason for most of them to interact with eachother. If only a small number of players on a team know things and the rest just stick to someone who seems to be proven trustworthy, only a small number of players are really playing.

There are several ways to avoid this, one is to set the game up in such a way that no player can safely "lead the village" by not offering them a reliable BG, or giving the mafia some way to kill them. Another important thing to do is force the mafia to infiltrate by giving them such of a numerical disadvantage that they will simply lose otherwise, and make their job easy as far as role PMs go at least. This keeps everyone on their toes and prevents too much trust/predictability from entering the game as well as giving the mafia something to do other than choose kills.

Giving people specific objectives in the game other than "you win with <team>" can help this to an extent, especially if that condition goes against the best interests of your team. This intruduces an element of doubt to even "confirmed" townspeoples true intentions.

Beyond those, what makes a good game? Having a load of expermental stuff makes things interesting, but can backfire, and maybe things like not giving out too many totally bland roles (can be boring), or overstuffing the game with high power roles.

On one hand, you have balance. You will want the game to be somewhat even so one faction doesn't dominate the game with ease. And on the other hand, you want to reward people for getting ahead. Viva la Mafia, for example, almost punished factions who got ahead of others by simply cutting their numbers even with lynches and all. Generally this isn't as much of a problem for forum mafia, especially not small ones, because there aren't as many kills, and it is usually hard enough to get a lynch in on an evil guy without even worrying which of the two evil factions he's of.

For main factions, usually something between 1:4 and 1:3 is what I do of a single village vs a single mafia. Like a standard RTM has 16-18 players with 11-12 villagers, 4 mafia and maybe a wolf. The wolf's kills are usually aimed at the village, but he can also hit mafia, and will want to when the game draws towards a mafia win. Many small games have two mafia factions for some reason, which is weird since it makes it harder to balance things...though usually both mafioso end up hitting villagers. But overall I don't get the obsession.

For the wolf...there are no set laws, but generally the wolf cannot win unless he has some way to falsify inspection, which is usually broken, so it has to be limited in some way. He should be able to kill more often and/or more effectively than any faction, and he's the #1 spot to rig players for since you don't want an incompetent player to use your surprise wolf role. He should also have ways of false claiming, both role and role name-wise...and it really isn't too much for a wolf to ask for a generic inspection, hooker, etc role.

The mafia(s) usually, by extension, ends up as a multi-man wolf in a way, though they don't have to be all great players, they don't need a mole role per sé, and they don't need to kill as often.

The village needs some roles that need to work together to be effective. Bodyguard, inspector, hooker etc are almost obligatory as otherwise it's extremely hard to build a trust base...I don't believe it can work out otherwise. You can balance how easy it is to "gather the village" through knowledge roles ("you know your lover is in this game", "you know there's 20 villagers, 5 mafia and 2 neutrals", etc) , announcers, etc...you can also make it harder for them with obvious ways. Sometimes in RTM I allow the mafia a one time BG Pierce so they can kill the "village leader".

Despite the fact that it's undesirable for overall game play, most village wins are based of a very small amount of people knowing what the deal is.

And neutrals...I don't like most neutrals because what they do and who they side with is very hard to take into account, even if you know someone's playstyle. This can be remedied by giving them a partial alliance (for example, they can win with 2/3 of the factions), or giving one of the alliances a connection with him (Bastian/Volke in FE Mafia). Generally I would recommend not having more than 1-2 non-wolf neutrals at most. In FE Mafia, I also made their votes not count.