A) Fantastic producers -- the production from these guys is phenomenal, and IMHO this is what really "makes the song".
B) Great at writing to a formula. The tunes are "catchy" but I cannot think of anything that would be considered really innovative musically.
There's no sin in that necessarily... it certainly takes talent to "know what works" and "know what doesn't"... and for the type of pop music they do (largely teen pop and other pop formulas), formula is fine.
I will say that pop producers like these guys tend to be really solid in the groove / rhythm department, in particular.
C) Great at artistic collaboration... big pop outfits now write in large groups. Knowing the way many musicians are, that's an accomplishment in itself. :) This probably contributes to the "solid song that isn't innovative" aspect... all the left-fieldisms of an artist are probably smoothed out. But again, this pop.
Overall the music product is a professional creation, designed for a market that always was and always will be half song-and-dance youth fashion show as one of the posters alluded to. :) Not a music style I care for personally.
But a popular one, and it takes something to do it as well as these guys do.