As an experienced Python dev who recently started programming primarily in java: You should build something big in a scripting language - probably ruby or python - the different style really will make you a better developer.
I think learning java made me better ad designing classes (I never explicitly used a singleton in python before) and helped a bit in my thinking about concurrency.
For python or ruby try to follow the common idioms rather than programming in a familiar style. I'd recommend avoiding classes altogether at first, trying to get as much done with the built-in data structures and functional aspects of the languages.
For me the thing I miss most in java is the interactive interpreter. I usually develop with an ipython instance open so I can test out different ideas and explore the features of a new package quickly. I believe that is why scripting languages are favored by hackers and startups.
I think learning java made me better ad designing classes (I never explicitly used a singleton in python before) and helped a bit in my thinking about concurrency.
For python or ruby try to follow the common idioms rather than programming in a familiar style. I'd recommend avoiding classes altogether at first, trying to get as much done with the built-in data structures and functional aspects of the languages.
For me the thing I miss most in java is the interactive interpreter. I usually develop with an ipython instance open so I can test out different ideas and explore the features of a new package quickly. I believe that is why scripting languages are favored by hackers and startups.