I think that karma is a useful feedback but only at a very approximate level. ...

... there's very little general pattern to how far one comment gets voted up compared to another even when they are very similar comments.

If that's true, then... what's the point of karma scores?

How about this: keep track of total votes behind the scenes, but only report whether the karma is [- -] for k<-5, [-] for -4<k<0, [0], [+] for 0<k<+10, or [+ +] for k>+10.