Most people think it’s the mechanics of trading. It’s easy to understand why they think that. There are indeed some odd concepts you have to learn for it to “make sense”.

But truth be told, the mechanics are pretty easy. Anybody can learn
them.

Many people think the biggest barrier is the scope of the market.

It is indeed huge, and the number of variables that can affect any trade you put on is vast. Trying to understand it all (and keep track of it all) can drive you certifiably insane in no time.