The most important thing to me is that the main riff of a song or the main chord progression is a big enough hook that it makes people want to move. Whether that means dancing, nodding their heads or just tapping their feet. This is the only criterior that I judge whether a song gets fully developed or not.
There are degrees of poppiness - sacharrine is unacceptable - I want my hooks to be intelligent and surprising. Not so obvious that you're sick of it after five listens. They should be immediate in a live sense that if you're a first time listener of the song you can still enjoy it, but it should reveal itself to you slowly over a number of listens so you only really understand the strength of the song over time.
What is a Riff?
A riff is a short musical progression that when played over & over becomes the main body of a song. Probably the most famous Riff in rock music is 'Satisfaction' by the Rolling Stones. da da dadadaa da daadaa followed by Mick Jagger's fantastic lyrics
'I can't get no/ Satisfaction
When I'm watching my TV
& a man comes on & tells me
How white my shirt can be
But he can't be a man cause he doesn't smoke
the same cigarettes as me'
Says it all really. That's the perfect combination of a killer riff backed by killer lyrics - everything rock music should be, rebellion, the threat of violence, sexy. You hear Satisfaction in the right environment and you know you're listening to the real thing.
What is a hook?
A hook is the term given to the indefinable characteristic in music that makes you want to dance. What seperates 'Hit me baby one more time' from anything else that Britney's done? Or 'Living on a Prayer' from other tragic hair band songs. They have the same chords as other songs, played pretty much identically to thousands of other tracks but for some reason when you hear these songs you want to dance.
This is what's meant when saying that a song has a hook. It grabs you & makes you want to move. The ability to write a riff with a hook is what defines a songwriter, great ones do it over & over - Lennon & McCartney - lucky ones stumble over them by accident & almost never manage to write another one - The Bodyrockers - and average ones might come up with something every now & again.