One simple thing that I've noticed that's missing in Arma 2 is the ability to delete or update Diary Subjects and Records by script. With this addition, Mission Makers will be able to create dynamically-updating notes.
And for me it means better development into a script I'm making that would create a list of Players on a side, sorted by groups, and then ordered by Group Index (As in the F1-F10 keys index). This would be a fairly simple script if I had the ability to delete or update my Diary Record, but since I can not within the confines of Mission Scripting (It is possible via the config.cpp method, but that would require an addon to be loaded), the Diary Record can only be updated once and when players leave/join the list would be left entirely outdated.
Like I said before, this addition seems very simple to the eye, but there could be a underlying reason why this hasn't been done yet, such as deleting persistent subjects like Notes and Units, though I would still like to see the ability to delete or update Diary Records.