When I was a teenage atheist is was mainly a renunciation of the Bible compared to scientific fact. Being young, I didn't get further past that concept; the conflict between those claiming the Earth was only 6000 years old vs. the scientific knowledge of the Big Bang, age of the Earth, dinosaurs, etc.
Agreed on religious extremists, and I include militant atheists in this, since they are pushing a belief system without evidence of that belief being true. They are two sides of the same illogical coin.