Vertical growth is what I feel ruins most PVP based MMORPGs. The individual gains too much power thus throwing off the balance of combat. "He hits for 1,000! I only hit for 750!" Compare this to lower level combat where someone would be hitting for 100 and someone would be hitting for 75. That is a tremendous difference in damage per hit at higher levels. I just can't fathom how hard it is to comprehend exponentially increasing power of a player ruins balance if the power is not kept in check.

Granted, the balancing of other statistics is also key, namely hit points. Lower the power curve(vertical) for games involving PVP while providing more play options(horizontal).