I saw Avatar on Christmas Day. That was my very first 3D experience!
Yes, the jellyfish scene... I can see why it was upsetting.
The scene that was upsetting to me was when I saw the "tree" getting destroyed later in the movie.
I really loved this movie because there is so much spirituality in the living of the "alien" Avatar people that aligns with my beliefs.
I also found the binding the tips of the hair braids with the animals/creatures concept quite interesting.