I didn't originally subscribe to this either, but am happy to provide input.  This seems to me to be a major issue, involving many aspects.  For one thing, having asked questions myself on Launchpad, and not having found the answer through Launchpad, I would say Launchpad itself needs a new button or two.  One would say something like "Forget It" (meaning it wasn't that important and can be forgotten) and another MIGHT be used to say "Found answer myself".  Another problem is that, as previously pointed out, older questions slip away and are soon forgotten.  These should automatically generate an email to the original issuer of the question, asking if he/she wants to continue.  If so, perhaps the date could be bumped up or some of the "better subscribers" could be notified about the question still being open.  (Those being the ones who solve most issues.)  Perhaps the question could be "renewed" (date changed to more recent date, bringing the question back up to the top).
     Personally, I believe that the karma issue brought up in the beginning is a bit of a problem for me.  If possible, perhaps Launchpad could track which type of issues each person is good at answering and credit them accordingly, maybe notifying those individuals of future questions on the topic.  At the very least, though, karma doesn't amount to much if it's just a number.  I would guess more people would pay attention to it if more karma MEANT something, more capabilities or trust when on Launchpad or something.
      In addition to that, seems to me a lot of people never come back to say whether their question was solved or not or provide ANY additional info.  Had one guy ask how to set up xsane(?) to use Tessaract (question 82273).  I spent days working with him and researching how to help him, and asked specifically if he would come back to tell us how well it worked.  I told him I set up gocr to work with xsane, which was easy, but was interested in whether Tessaract would work better.  He never returned.  I don't know if I solved his issue or not, but would think this would have been a nice type of thing to pass off to, say, Ubuntu Magazine (or some other).  Perhaps they could have researched whether my answer was sufficient to set it up, how to do it if I was wrong, and whether it worked any better than gocr or not?  Perhaps if they are interested in such, the recognition of being mentioned in a publication might be more inspiration for some than Karma?
     It also occured to me that tracking those who have problems and solve them, asking them to help people who have similar problems, would provide a type of mentoring.  As they learn, they take care of problems they have learned how to solve.  Copies of their original questions (thus, who helped them) could be provided to them so they could reassign to whoever helped them and the "elder user" could add in "Good job, sounds like you learned something.. I would also like to add...." so that a true family community would be fostered.
     I was also disappointed to find that learning about PPA's and packaging and uploading software that wasn't available in the usual places isn't worth much more than just participating.  More difficult journeys like that should be worth more to encourage such.  Perhaps in line with increasing trust and capabilities, if a software package in a PPA worked well and turned out popular, it would be added in to the main repository (yeah, yeah, checked over first, etc.) under the control of the original writer.  The one I worked with wouldn't qualify because I just debianized it and can't get in touch with the original writer, but as an example of how the Karma could be worth more....
      Anyway, just some rough ideas I got out of looking this over.  Community, etc., are the ones who are important - let's hear what others have to say about it and whether any of this is worth developing.