Library support is a big issue, for now, until library developers develop their support for Python 3.x.
For example, here are some popular libraries that you might be interested in learning, that do not yet support Python 3.x:
- NumPy and SciPy
- Django
- wxPython
- PySide (free alternative to PyQt, see below)
Here are some libraries that do support Python 3.x:
Libraries with support in-progress: