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Many PvE games that bolt PvP or some pale imitation of RvR on suffer from combat mechanics that do not lend themselves to PvP. PvE and PvP are so different that a combat system focused on PvE is inevitably going to be subpar for PvP.
The two aspects I feel are of prime importance are pacing and time to kill, which are somewhat related. There are of course other things, such as control powers, etc., but those are not as fundamental and can be changed as balance requires, whereas altering the actual pace of combat and the time to kill is much more difficult as that involves altering the entire combat system. Thus, it must be done right from the beginning.
Since PvE is predictable, as enemies follow a script, which ranges from "fight back" for a common enemy to "follow a hokey WWE script complete with fake deaths and extra combatants added at set intervals" for raid bosses, combat can and should be quite fast-paced. Since enemies do not maneuver much, nor do they employ clever tactics or actually coordinate in packs, quick combat makes sense. It helps hide the tedium and make the difficulty a bit higher, though not much. The goal is to simply plow through enemies as quickly as possible, spamming abilities as quickly as possible, to attain some sort of reward, like a level or an item.
Combat rounds, where one combat round is loosely defined to be the length of time between an average weapon swing, should be short, perhaps 2 seconds at most. Ability cooldowns should also be brief. The time to kill a normal enemy should be perhaps 5 or 6 combat rounds, or around 10 seconds.
In PvP/RvR, this all falls to pieces. Suddenly, there is no script, and your enemies are (theoretically and hopefully) intelligent and capable of advanced tactics and coordinating in packs. Combat is no longer about spamming abilities as quickly as possible against a stationary foe, but rather about making smart, moment-to-moment decisions to keep yourself and your allies safe and in a position to win.
I feel a slower pace ultimately makes this a better experience, coupled with a much longer time to kill. Slowing down a combat round by necessity makes one invest more thought in their current action, and also allows them more time to decide a proper action. Also, by increasing the time to kill, it increases the longevity of battles and makes it more about thorough strategy and tactics rather than an assist train or some one-shot wonder combination of abilities that decimates an enemy before a reasonable defense can be made. That is not fun for either side, in my opinion. What is fun is a good, long battle where victory rests not on a few short moments, but on a series of good decisions. I do not mind losing battles like that at all.
Basically, I hope the combat in Camelot Unchained tailors itself to PvP/RvR, since PvE is not even an issue. This will allow them to make all of the right choices and make that experience perfect, or as near as possible. Dark Age of Camelot, for example, did very well. Combat was paced more slowly, and the time to kill an enemy was somewhat reasonable. Building on that will be great, going backwards to spammy combat would not be.