IME "nullable by default" is one of the biggest sources of runtime crashes in high level OOP languages like C#, which is a shame because it really isn't that difficult to statically eliminate the vast majority of them, especially when you're sort of hand-waving the semantics of your language anyway and don't require it to be super rigorous...
I definitely agree. After I'd been learning Haskell for 6 months and then wrote a program in Java & C++, almost the first thing I did was code up a generic Maybe<T> class in each language. It is so much clearer and more obvious to _explicitly_ have no value (Maybe.Nothing()) as opposed to _implicitly_ having no value (null). Now I find my Java & C++ Maybe<T> class indispensable when I am programming in those languages. Eric