- No pattern matching. This alone basically makes the rest of this list just piling on; once you have 'gotten' pattern matching, Java (and, to be fair, most other popular languages) becomes intolerable.
- Lack of even halfassedly reified generics means you're throwing Class objects around everywhere for no good reason. (Scala adds manifests as implicit parameters and it's not as good as .NET but it's a hell of a lot better than Java.)
- Actually, let's just go with "the type system in general". For being built upon the same shitty tools as Java, they do a lot to provide an environment where thinking about types isn't prohibited.
- Inner classes being used for nontrivial things (see also: all of Android, saved only by AndroidAnnotations; all of Swing, saved only by never having to use it).
- No comprehensions. Nuff said.
- Null by default. Option types are both safer and easier to work with functionally than imperative null boilerplate.
- Nonfunctional control constructs. For example, 'if' being a functional construct makes tons of code simpler and easier to read.
- No default parameters. Dumb. Whenever they get them I'm sure it will be something less expressive than Scala's (something like "must be resolvable at compile-time") because doing otherwise will be hard.
- Braces everywheeeeere. The ability to go "def foo() = someFunc(localVar)" is awesome. Less bullshit, more code.
- Interfaces suck when traits exist. The idea that I need to write wireup code when I have getBar and getBars methods (because any sane person will implement getBar as a hat on top of getBars with a singleton list) is utterly stupid.
- Nonfunctional core APIs (where's my map/filter/fold?) that think mutability is a good thing. Guava tries to help, but it's bolting onto a fairly stupid List/Iterable core pattern.
- Reference-reseating-by-default means doing it right adds foolish 'final' keywords everywhere.
- Private visibility (or, to be specific, not-really-private, WTF?) by default means you will be splattering 'public' everywhere, too. This and the last two combine to make objects full of weird action just for funsies; if everything's immutable by default, the cases where you need 'private' to not blow up the world shrink immensely.
- Keywords and methods where operators make sense. List.add() is derpy. "T extends SomeClass" is a traveshamockery. We're programmers, computer science should not be that foreign, there are mathematical symbols to express this in type theory--so use them, as Scala does (I'd rather they used "<." instead of "<:", but I understand why they don't).
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Scala is complex. Nobody said it's not. But it's complex because it's expressive. It's a predictable language with expressive semantics that doesn't make you do stupid shit to get things done. The tooling has reached a point where I no longer notice it and while it is slower to compile than Java, so is C++ and that doesn't bother me either because what I get is better at what it does.
I don't much care for the look of Python, but it's better and more suited to some tasks and so I get over it and use it. You may find Scala "ugly" and I'm sure you can find plenty of entrenched Java developers to agree with you, but outside of the epistemic Blub-closure of the Java world it is rapidly becoming understood that Java is better at nothing by design.