As far as I understand, current Bugzilla allows people to change only their bug reports and only while it is "NEW". I think the following capabilities are also necessary (most important first): 1) Be able to add/remove my address to/from the CC list of any bug. This would help people who experience the same problem or just curious. 2) Be able to add additional information to bug reports. That will allow people to give you some more specific information so that you can find and fix the bug faster. That would also allow expert users to explain why the problem reported by a novice user is not a bug (so that you do not have to spend your time doing that). 3) Be able to mark my bug report as "FIXED". That would allow a user who have realized that his/her problem happened because he/she was doing something wrong to close the bug.
#2 is in place, the others are on the "TODO" list
#1 and #3 are still not implemented in Bugzilla 2.1rC Another missing functionality is the ability to reopen the bug (many bug reports have comments from RedHat developers saying "try this fix and if it does not work, reopen the bug report", but Bugzilla does not allow to reopen a bug report) Also, it would be nice to be able to change the "Version" field (when someone upgrades and them verifies that the bug is still there). Well, actually, it would be nice to be able to change any field :-)
users can reopen a bugs, and a generic "add/remove" is in place for the cc list.
I think that #3 should be handled by adding a "never mind" comment, because a user may have stumbled across a true bug but have come up with a work around in the mean time and lost interest in the true fix. Besides, the incidence of "never mind" bugs has been low enough that this is not a big issue at this time.