Magic Duel
This game promotes a non-spoiler attitude. Meaning, no player is allowed to tell you how to solve a quest, defeat an NPC, what this monster's stats are, etc... You develop your own strategy and find things out for yourself. It also greatly promotes role-play. Hints are ok in some cases. SOME.
Players actually CREATE some of the quests and can GIVE A REWARD. If you have been part of the game for some time, you begin to have a say in how it expands. There is one player that is about to have his home/smithy actually integrated into the game. The house is already there, just needs an interior coded and drawn.
There are some premium features available, but like most games they are not necessary. The plus side to the premium features is that EVERYTHING is only 1 dollar, regardless of it's power. The downside is that you have to have had so many active days (days that you actually logged on) or stats in order to be able to purchase about 60-80% of the premium items. I think this is mainly to prevent rich noobs from getting some powered up weapons and annihilate everything.
It's a browser game, where every npc, monster and 'room' of the map is hand drawn. Creates a pretty nifty atmosphere. Also, if you are a good artist, they will pay you for your work. So you can become an even bigger part of the game. More details inside.
That's all the cool stuff I can think of at the moment without spoiling anything. This is all stuff you can find out right away, and isn't related to story or quests or nuthin.
Remember, if you join, try to think quick about your background and your role in the game. In some cases, how well your name fits with your role helps too.
Summary: Medieval fantasy, heavy roleplay, spoiler-free, and the players can create quests and new aspects of the game and decide how it expands.