I think the true test of a pop song, for me, and I've talked to a lot of other writers about this, is you take your demo, you pop it in your car and you drive down Sunset Blvd. to Santa Monica, and that's the Hollywood car test.
Melody is the single most important thing to any song, period. I don't care what anybody says, it trumps everything. Not because that's my opinion but because I think it's actually indisputable fact. The human brain retains melody easier than it retains words. It's that simple.
If you're talking to an architect, he can look at a blank piece of paper, and once the initial design is there, the formula kicks in. Each room should have something unique and different about it - much the same way that in a song, every eight bars or so, a new piece of information should be introduced.
I think the best songs are being written by the very under-stated, under-appreciated indie artists. The thing that separates them from mainstream success is they either consciously or unknowingly refuse to deliver on a big chorus.
With iPhones, nobody has an excuse for writer's block. If you're at Whole Foods getting your green tea extract and you have a melody, you just drop it into your voice memo and save it for later.