I agree it's not hard once you get it, but the main thing is people don't get it. It's unintuitive. That's not to say the model is wrong (I think it's correct), just that there's a huge gap between what people think git is doing and what it's actually doing and so they end up fighting it to do what they think it should do.

I had a coworker explain that he did a merge and something unexpected happened to his branch. I had to explain to him that what happened was totally expected and that his understanding was what was wrong.