Having said that, a song built only on them can sometimes sound somewhat … tame or safe… the kind of thing which would make grandma clasp her hands and say, "Aw, isn't that nice!" I think what is missing is key changes. The key change in -- to pick one of around 100,000 examples - Penny Lane is what makes it special. I'm sure everyone knows of of pieces where a key change "kicks it over the top".
Is changing keys a matter of trial and error, or are there any (even rough, non fail-safe) guidelines for, for example, if you are in the key of C, which key would be good to change to, depending on the mood of the piece?
(Please, if you happen to be anti-shortcut / anti-guideline- let's not have that argument here - think harmony