1. mass appeal. webcomics like to tout themselves as being underground underdogs in a world where newspapers censor and want clean, pristine comics. but that doesn’t excuse webcomics from not making a strip that can still appeal to people of different groups and lifestyles.
2. universal humor. some strips just lack humor and when they try it most certainly is not universal.
3. truth to the humor. jokes aren’t just funny but they have a level of truth to them which in the end creates the mass appeal and universal humor.

So far, Piled Higher and Deeper is one of the few webcomics I found that genuinely meets those qualifications which is why its one of the few webcomics to actually see actual book compilation publications and even a few newspaper printings.