Just a short list, not too bad.
- The __future__ module
- The print function
- Integer division
- Unicode
- xrange
- Raising exceptions
- Handling exceptions
- The next() function and .next() method
- For-loop variables and the global namespace leak
- Comparing unorderable types
- Parsing user inputs via input()
- Returning iterable objects instead of lists
- Banker’s Rounding
http://sebastianraschka.com/Articles/2014_python_2_3_key_diff.html