The biggest one is types -- students think in terms of meaning, not types, e.g. "this my variable contains first name", but they don't realize it's a string. Or "this is products in checkout cart", but the variable is an integer, so it's products count, not a list of product. Once students get idea of getting an item of a collection and getting a field of that item, while watching every type along the way -- that's a major breakthrough during learning.

The easiest to learn would be a language where you have to define all variable types on top, like Pascal (a language created specifically for learning). But Java is still better than Python, as it doesn't even run wit messed types.