You're right about the Common Rooms. The other places you can edit , are
the Common Rooms, the Harry Potter Fan Fiction forum, and the Other Fan Fiction and Writing forum.
There were a few other places that I had missed out and which still had editing enabled but I've turned these to Moderator-Only editing now.
The main reason editing is currently turned off is to ensure no-one posts any nasty messages and then tries to hide it later by editing it out.
Once the big "Olympus" upgrade to the board is in its final version, it looks like there may be a time-limited editing facility available wher you'll be able to edit within a set number of minutes after posting, but after that period has gone the editing facility disappears. This is exactly the type of thing that would be the ideal choice, and if this facility is available, editing will be turned on throughout the entire forum.