Personally I'd like to see the y-axis of the 'Default Category Permissions' expanded to four rows, 'Own Discussions', 'All Discussions', 'Own Comments' and 'All Comments'. Admittedly that would allow for some pretty weird combinations, but it would put control firmly in the hands of the administrators to set the access control up as they see fit.
However, in the dashboard there is now a setting, under Forum -> Advanced called Discussion & Comment Editing that will allow you to set for how long a user is allowed to edit their own posts, so long as you don't mind the setting being the same for all categories.
Forum -> Advanced
Discussion & Comment Editing