What about the times of day/month/year that a druid's power fluctuates?

A ranger's abilities, or lack thereof, in certain places are fairly constant. Druids are more... cyclic.

It hardly seems worth it to toss a 100xp penalty on them just for game balance, when they seem fairly balanced in that regard as it is. You're also not taking into account that they are an empowerment class, rangers aren't. I think it's like trying to compare apples and oranges... or at the very least, cherries and loganberries.

*shrug*