There is also a new conf.json option "pistonProtectionThroughDenyBuild" (default true) which can be disabled to turn off piston protection. This option is available due to the (untested) potential that a world with many pistons constantly repeatedly firing could result in additional lag from these piston protection events
NOTE: if Essentials is too old, this error will occur at server startup (sadly impossible to catch or otherwise handle due to how Bukkit loads plugins):
java.lang.NoClassDefFoundError: com/earth2me/essentials/chat/IEssentialsChatListener