Regarding barbarians:
I think there should be more animals, so that taming them isn't just an early game rush affair? Tigers might be a bit too painful, because there's no easy way of staying out of their reach, like with Bears. Orthos (5str superbarb with axe) is really difficult to kill unless you rush to (and find) copper, particularly after the AI has spoonfed him promotions... I think I might have seen him with Shock!
Raging barbarians can be really painful - any map with default or less players will crawl with barbarians. It is fun the first few times, but they might be a bit to agressive if a lot of wilderness remains. I actually built a wall of Elven Archers in one game, around most of my rather large empire, excluding a few AI borders, because they were crazy about pillaging everything I had - a massive wall so they could simply not enter. It was a small map with raging barbarians, and they had wiped out three of the six AIs, so I was left with barbarian hell.
Being a buildaholic, I absolutely adore the Druid's "Nature" ability, but I wonder, is there any way for "Nature", "Cure Disease" etc to not destroy improvements?