Not just catchy.

Pop is a mixture of the most "appealing" elements of rock/urban music. Therefore, it appeals to a wide variety of people since it has the most appealing elements of that type of music. Sometimes they mix it up with techno & dance as well.

It's usually identifiable by 3 things:

1. Very little or no meaning
2. Beats that are "dancy"
3. Hooks that are catchy in sound terms

When i say catchy in sound terms, i mean they SOUND catchy. Underground rap can be catchy cos of the rhyming, flow etc.. but pop is usually defined by sung vocals that sound catchy, as in the way that they wanna make u dance, if you know what i mean.

That's why Nelly for example is considered pop. He has beats that are make chicks wanna shake their asses, has practically sung hooks, and his songs have no meaning past the same thug love crap that appeals to most teenagers. He's not pure pop, since he technically doesnt sing his vocals, but he comes very close, with his high pitched voice changes etc.

Dont get me wrong, mainstream rap isnt always that bad. There are rappers that can make catchy pop songs but still have meaning (nas etc.) and decent beats that arent made simply for selling.

Finally, since pop-rap is essentially made to sell records, there is little need for lyrical complexity or meaning, which explains the lackluster content and weak lyrics in most mainstream rap.

Hope that helped.