People say I overthink things, but my answers may vary based on context.
For example, question 1. Do you mean as a listener? As a collaborator? As a musician? I'm generally not into commercial artists because I get bored with the formulaic chord progressions. I guess, though, when I'm listening to "songs" (i.e. music around 3 minutes that has strongly identifiable common structures (verse, chorus, bridge, etc, regardless of genre)), I look for things like personal embellishments, key changes, clever (if understated) ways of going beyond the confines of form, anticipation, release. 90% of the time, though, I'm listening to production choices.
[will add to this and expand later].