I've been itching for a while to learn a new language. I cut my teeth on PHP, though I don't use it anymore. I primarily program in D and Python now. I feel that these languages complement each other well, because D is close to the metal, statically typed and bleeding edge, while Python is more dynamic, interpreted, more mature and has better library support.
I also know C pretty well (mostly learned from using the C-like subset of D), have some knowledge of C++, and have written a few very small one-off programs in Perl and Java.
The key things I want in a langauge are:
A language that significantly expands my horizons. I don't want to learn something that doesn't teach me anything conceptually interesting relative to D and Python.
A practically oriented language. I absolutely, 110% refuse to spend any effort whatsoever learning a language that feels academic and emphasizes purity over practicality.
Something that is mature, modern and well supported enough to use for real work. This doesn't necessarily mean large production projects, but at least serious prototypes and research code.
Good generics, templates, duck typing, type inference, or something similar to avoid the rigidity of plain old static typing is a must.
Terseness and readability count, though these are admittedly subjective.