It would be useful, yes. However when this sort of thing is posed to the LJ developers it normally doesn't get much attention since there seems to be a view of "if its public then anyone can see it if they log out".

Anyway, to get it working you'd have to do something like have a meta-friend which is "Public" - i.e. anyone - and then do checks along the lines of:

• If the viewer is not logged in and the "Public" meta-friend is in the friend's group then they can view it
• If the viewer is logged in and they are not on your friend's list then treat them as someone who isn't logged in
• If the viewer is logged in and is on your friends list then do the usual friends groups check

Thus you could create a custom group which would contain the "Public" meta-friend and then exclude anyone on your friends list who didn't want to view quizes and the like.

The other alternative is to create keyword lists for posts - something which could contain words like "quiz", "meme", etc and then update the web client, flat and XML-RPC interfaces to support this. Then you could effectively kill-file posts you didn't want to see. This would require work on the part of the people viewing the journal so it may not fly; people would also have to remember to associate the right keywords with the right posts (that said, you'd still need to do this with the custom group solution above).

You could always make a suggestion; see here for how.