I am not a language designer, nor a researcher in type systems. My concerns are purely pragmatic. I want usable languages, ones where doing the easy thing also happens to be doing the right thing.
Even today, I see a lot of code that handles exceptions poorly (whether checked or unchecked!). Even after 13 years and some trillion words of books, most Java developers barely understand when to synchronize code. (And, by the way, I now believe that there's only one book on concurrency you actually need.)
I still recall the agony of converting a large C++ code base to const-correctness. That's something that you can't just do a little bit. You add one little "const" keyword and sooner or later, you end up writing some gibberish that looks like:
const int foo(const char * const * blah) = const;
I'm exaggerating a little bit, but I bet somebody more current on C++ can come up with an even worse example.
That's the path I don't want to see Java tread.
On the other hand, Robert Fischer pointed out that type inference doesn't have to hurt so much. His post on OCAML's type inferencing system is a breath of fresh air.
There's quite a bit of other interesting stuff in there, too. I particularly like this remark:
What the Rubyists call a "DSL", Ocamlists call "readable code".
I'm still working on wrapping my head around Erlang right now (it's my "new language" for 2008), but I might just have to give OCAML preferred position for my 2009 new language.