I've played around with addons like Karma or NoteIt, but none of them was what I was looking for. And every single one of them was ancient and apparently unmaintained.
A real social interface would allow me to add private notes about any character I encounter. I do a PuG with someone - I add a note to that character. Someone says "thank you" when I do a drive-by buffing and even buffs me back - another note. Beggar in Orgrimmar - note it. Stealing my resource node from right under me while I fight a mob - note. Horrible tank that thinks two-handers and Deathwish are the way to go - note!
The notes have to be shared between all characters on the same account and server. An idiot I met on my main stays an idiot when I log on one of my alts. The note should be displayed on mouseover and when shift-clicking the player in a chat window.
And in times of server transfers and name changes a pure matching by name isn't enough. The race and class of the character should be recorded automatically, so I notice if I meet a completely different idiot just with the same name than the idiot I met a year ago. And the guild of the characters should be stored. While I don't judge a player by their guild tag, it does give a hint about the quality of the recruitment.
Since none of the existing addons fit my requirements I've started to tweak the simplest one, NoteIt, to my needs. NoteIt provides a simple notepad ability where you can assign notes to a player or item and have it displayed on mouseover.
So far I have extended it with an "info-stamp" that adds some extra info whenever I save a note, like the current time, the name of my logged on character, and the race, class, and guild of my current target.
So when I target someone and type /ni or /noteit then an edit window pops up that allows me to set a note, for example "repeatedly jumped into my fishing bopper". When saving it some data about my character and the targeted player get added like this:
-- Kirana - 2008-03-17 - Thunder Bluff:
-- male level 17 orc warrior
repeatedly jumped into my fishing bopper
Not perfect, but works well enough to be useful to me.
The next steps will be integration into the chat window so that the note appears when I shift-click a name in chat as well as handling /who queries when adding notes for players that I don't have targeted at the moment.