Years ago, when I would tag-along with my wife at her church, I got some perspective on how these mega-churches work. (I don't know what the total attendance was at its various locations, but I'm guessing it was in the high four-digits?) At least for this particular church, the draw seemed twofold:

1. They were very oriented to young adults (say, <40 years old): modern music, chic building/space design, coffee-shop feel (especially because people would regularly come in with their lattes), and the amazing perk that they had numerous rooms for kids of different ages. Kids were treated very well and had so much fun in the bouncy houses or whatever, and parents could really focus on whatever the message was that day.

2. Critical mass. Being around more people really gives a feeling of belonging, and I think it's pretty well-established that the big crowds, swelling music, etc., really does elicit an emotional response that some people interpret as the holy spirit while others simply think of it as a warm, wonderful feeling you get going to church. That feeling is definitely self-reinforcing for attendance.