for me the biggest thing was the whole strict enforment of nullability - doesn't always play nice with existing java code, builders (essentiay abusing the anonymous function syntax) and delegations.

Its just very odd when you see some code from someone else who really goes to town with these things.

Also just in general its very different approach from Java where you have to do everything with very basic but easy to understand tools while Kotlin gives you a giant toolbox to play with, but where many tools have a certain learning curve.

But in that toolbox there are some gems. My favorites being extension methods null safe calls and pattern matching with when.