I'm not familiar with how Avalon does it, but you could write a book on Achaea's PvP rules and the other games by IRE are almost as complex. That's not something I want for Encarnia. I'd like to develop, for better or worse, systems that somewhat auto-regulate PvP so that we can spend our time on making the game better and not sorting out squabbles between players.
Here's my thoughts on this so far.
1. A "feuds" system. Think of it like a declared duels system, where players can attack each other through mutual assent, only it doesn't automatically shut off after one person dies. A feud between two players lasts for as long as they both mutually agree to PvP with each other. If someone agrees to a feud and then someone else finds a way to "cheat" in their feud, that's not our problem. Don't agree to feud with people who don't follow agreed upon rules.
Making feuds work would mean adding in the ability to selectively mute some forms of communications from players, which I intend to get to relatively early some time after the alpha launch stage.
2. Limited "free form" PvP. It would cramp people's style if they can't spontaneously attack anyone ever, but how to make sure that's used appropriately? Strictly speaking we can't, but I'm imagining that you would have one person you could attack every 48 hours or something without having a declared feud. Some people would probably abuse this but I imagine it's better to have than not have, since if you know you can mouth off with zero repercussions by just not accepting feuds, some people would work that. I'd rather have some bullies that occasionally attack people than some trolls.
3. Clans. Another thing I want to do early on, perhaps even before guilds, is to let players make their own clans. Clans would also be able to feud with each other, which should lead to a form of player wars. Players could likely join more than one clan.
4. Bosses/nightmares. I'm imagining that as they're online, a player accumulates "nightmare points" which can be spent to log into a nightmare character. The nightmare character can take on a certain class, which cannot leave a bashing area, and act as the "boss" of that bashing area, such as the aforementioned "titanic rat" in the giant rat newbie area. This is an ambitious system but if it works out I think it would be pretty cool.
5. Berserk Fae. Originally I was planning for people who use too much magic to become berserk Fae, this would have made them open PvP and also let them attack people. But I don't want trolls going berserk on purpose and attacking people. So probably they will just die and be replaced with a hostile NPC that shares their description. Maybe at some point in the future though this could be revisited.
6. Cities. This is way down the list, but some day there might be more cities than just the tower. People could sign up to be soldiers and/or partisans of their respective cities and attack each other at will. Soldiers would be open PvP during a declared war, partisans would be open PvP to each other all the time in certain "contested areas" but not everywhere since that would probably get fatiguing.
Subject: How will we handle PvP? 
