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MMORPG.com columnist Scott Jennings writes this look at two ways that including PvP in your MMO can end in complete disaster.
Nothing starts more arguments on MMORPG-focused message boards and blog postings than the topic of Player vs. Player. Call it PvP, RvR, PK, whatever you like, it is the subject that will rouse more passions than any other, bar none. For some reason, some people really like killing other people. And other people really don't like being killed. Go figure!
I've written several blog postings about it myself, as has anyone who's written about MMORPGs for more than fifteen seconds. Invoking PvP is the Godwin's Law of gaming discussions - in any discussion of game design, the longer the thread, the probability that someone will introduce PvP as either the reason for its subject's success or failure approaches 100%.