1) Time - studies have shown that the perfect length for a pop song is 3:26. That is the length of time people stay interested in a song before tedium sets in and they tune out.
2) Note progression - generally a three note cord progression or keyboard stroke repeated enough throughout the song keeps it running in a person's head. If a popular vocal concept is introduced with it, people will be singing it and whistling it as they do things.
3) Theme - the theme generally is love, lust, or dance. There are exceptions, but those are the big three themes.
4) Beat - the song, particularly now in the age of clubs and thumping car stereos, needs a good beat that you can feel almost physically. A beat slower than 4/4 is unacceptable in a pop song. That makes it a ballad or just depressing, not a pop song.
5) Unique quality - you need to differentiate yourself from all of the other acts, but only slightly. Pop does not reinvent the wheel, in fact, it only redecorates the same wheel that has been turning for a long time now.
6) Looks - the artist needs to be hot. Pop music generally does not accommodate less-than-hot people.
7) Talent - the more talented the artist the less hot they need to be, but if there is no talent then there is no product.
8) Frequency - if you want money you need play on the radio, performances on shows, and downloads, downloads, downloads. The more people hear it the more they want it.