OK, here goes;
In Peanuts, Schultz tapped into the fact that we all have been, and know, children.
In QC, Jeph has tapped into the fact that we have all been (or are, or (in the case of the youngsters out there) soon will be), and know, twenty siomethings.
Both comics are almost entirely about social interactions within these groups.
Both comics have a wacky non-human instigator.
The protagonist of both comics are, by their own descriptions, wishy-washy (OK, Marten's never used those terms, but come on...)
Both comics deal on a regular basis with universal aspects of being human - insecurity, love, depression, friendship and angst, to name a few. True, the Peanuts crew rarely uses such terms, but they're dealing with these things nonetheless. There's way less sex in Peanuts, of course. But most of the issues are still there.
I dunno, these just came off the top of my head. There are other comics that may cover the same territory, but it's the fact that all these are constantly running as subtext, with only occasional bubbles to the surface that really cement it for me.
Just like in real life...
Skewbrow: Worry hat?