Personally I've never understood why druids don't have an exp penalty. Maybe that's because I've never played one, but to me, the class should have one.

I find them deadlier in the wilds than a ranger, and a ranger has a penalty. I find them MUCH deadlier outside of the wilds than a ranger.

I might be speaking from non-druid ignorance here when I say that saying a druid is vulnerable for two minutes when he does plant growth doesn't mean a lot if he grows all his plants at a time when sylvans outnumber everyone else, and are protecting him.