In real life though, barbarians were quite week. Unadvanced, and without strong weapons, tribes were called "barbarian" because they were defeated. It was those that deated them that called them "barbarians." Therefore, barbarians are inherently weak. In the past, I have considered this, by thinking "Barbarians don't have to defend their homes, so they should have high attack, and weak defense." But, barbarians already get an advantage for not having to defend anything: they don't have to defend anything. Although they have to defend their minor huts, they do this with one unit.
What I would like to see in Civ4 is the abolision of barbarians, and the instatement of "barbaric" tribes that go on raiding and ransacking the rest of the world. This would be far more realistic. Perhaps some civilizations could focus primarily on military, and not science, or growth. These civs would attack the more advanced empires of the world, and wreak havoc. However, the rest of the world could defeat them by technilogical advances. This could easily be done in Civ3 with a modpack (utilizing flavors, and build orders), but cues would not be as strong in Civ3 as they could be in Civ4. I also think that this should be "standard." Obviously, some people will disagree, but I think that there must be some people who share my opinion.
Please read this post. I myself hate long posts, but I strongly encourage you to listen to my idea. You will either love it, or hate it; either way, you will want to respond.