A great song doesn't attempt to be anything — it just is.
When you hear a great song, you can think of where you were when you first heard it, the sounds, the smells. It takes the emotions of a moment and holds it for years to come. It transcends time. A great song has all the key elements — melody; emotion; a strong statement that becomes part of the lexicon; and great production. Think of "Bohemian Rhapsody," by Queen. That song had everything — different melodies, opera, R&B, rock — and it explored all of those different genres in an authentic way, where it felt natural.
When I'm writing a song that I know is going to work, it's a feeling of euphoria. It's how a basketball player must feel when he starts hitting every shot, when you're in that zone. As soon as you start, you get that magic feeling, an extra feeling. Songs like that come out in five minutes; if I work on them more than, say, 20 minutes, they're probably not going to work.