1. A properly prepared and knowledgeable player should be able to defend him/her self. If you can kill another player from start to finish without them having any chance of defending themselves *Assuming they are properly using all the tools available to them to do so* then this is imbalance and requires fixing.
Example- Stunning a player to death from start to finish with no way to break or block the stun and no way to fight back.
*note* if there is a way to defend your self available in game and you CHOOSE not to use it, that is your fault not an imbalance.
2. A properly prepared and knowledgeable player should be able to kill another player *Assuming they are properly using all the tools available to them to do so* if this is not true then this is imbalance and needs to be fixed.
Example- Players achieving 100% damage immunity, or enough healing that they are in actuality unkillable by any means would be considered imbalance.
*note* if you go into combat with a practice weapon and can't kill someone that is your fault for not properly preparing not an imbalance.
3. Players should not be able to kill another player who is properly using all the tools available to them in a single hit be it via spell or weapon.
*note* This is the most variable rule, i think we can all agree that dying to 1 hit is a bad thing but exactly how many successful hit's is required to kill another player can vary a great deal however this is something that should be determined by the shard owners.
*note2* If you go into pvp or pve wearing no armor and get 1 hit killed that is your own fault for not properly preparing not an imbalance.
There we go, i tried to keep the rules relatively simple but far reaching. These rules work especially well in ultima because of the way skills work, you are not restricted to a single play style or skill set, all options are available to you. If you choose not to use them then you will lose to players who do as you should.
These rules give more power to the players to determine what is effective and what is not, high level pvp and pvm has always and will always be about using every tool at your disposal to maximum effect if you choose not to use these tools then you should not expect to be as effective as those who do.