Drupal :-)
The system we used to use for writing On: was called SlashCode and it was badly maintained, over the top and complicated to use.
Drupal is much simpler (for us and you).
I have brought across all the old user accounts, stories and journals (which is what SlashCode called what everyone else called blogs) - however it will take some time for things like URL links and categories in the imported items to be sorted as they have to be done fairly much by hand :-/
We'll also be updating the looks a bit at some point too, and making it work better with the Gallery (which currently works but doesn't look too nice).
We hope you like the new site as much as we do - please check your account settings over, just in case.