World PvP: You cannot attack that target

Would it not be possible to mark characters (say x levels under your own) in gray, and if a player over the limit of x tries to attack they are presented with the message "you cannot attack that target"? (Similarily, a player x lvs under gets the same message if trying to attack a target x lvs over).

It’s possible to do that, but it’s just something we decide not to. PvP realms have been here since day 1. Players know what the rules are before they join a PvP realm, the environment can be frustrating at times but it can also be a lot of fun and very challenging, that’s why players they go there.
I think the difference between PvP and PvE realms was felt much deeper in the old days of vanilla. That was when world PvP was probably at its peak, places like Tarren Mill, Southshore and Tanaris were truly a permanent battle zone. But one has to realize that the game evolves and while we enjoy world PvP, we also enjoy giving players more options.

So what happened? Well for starters, battlegrounds were introduced, so a big chunk of players that was constantly doing world PvP immediately switched to Battlegrounds, these were purposely made for PvP, it’s just natural that they become more appealing and take away some of the ganking going around in the world. Even if this wasn’t probably a complete switch, it certainly reduced the average amount of players out in the world looking for PvP. Is this a bad thing? That depends on whether you prefer world PvP or Battlegrounds.
Then we had some more game expansions and in addition to the classic Battlegrounds we got Arenas and even more Battlegrounds. We also introduced quality of life (controversial, we know!) queuing systems where we let people instantly teleport into dungeons/raids without risking their necks out there in the wilderness.

Without getting into much detail, the point is, as time went on, the amount of players out in the world got reduced, while the amount of players inside cities increased, this created 2 issues: one, it increased the likelihood of low level ganking to occur, since levelling players tend to quest and they have to be out there in the world to do so; and two, it decreased the chance of someone high level being around you to defend the members of his own faction.

So, yes, world PvP decreased significantly but we’re trying to turn that around with Mists of Pandaria.
My whole point is that, between vanilla and Mists, world PvP changed so much that making the choice between being on a PvE or PvP realm might have stopped being as hard to make as it used to be, but the fact remains that PvP realms were always meant to be about freedom, allowing factions to feel that they are truly at war, how can you feel that if you suddenly have restrictions on how/who/when you can attack?

Personally I welcome this change of direction with Mists but I look at it very carefully as this isn’t vanilla anymore and there a lot of variables that we need to pay close attention to if we want to make world PvP as entertaining as it once was. I’m talking about faction imbalance on certain realms (where possibly CRZ tech might be able to help out), flying mounts as easy ways to escape danger, and lack of the “I’ll get my revenge soon” feeling with CRZ possibly reducing the chance of the same players seeing each other again.