I can't speak to every megachurch, but a lot of Catholics who are drawn to them say things like they are taught to have a personal relationship with Jesus Christ there (why aren't they getting this in their Catholic Churches, you can't get more personal than recieving Jesus in the Eucharist). They say things like they weren't encouraged to read the Bible when they were Catholic (why, when the Bible is the book the Catholic Church compiled). People don't know simple things like why Catholics pray to saints or why Catholics have Confession and they know Catholics are easy bait for conversions since they don't know these things. 

And just as bad as the prosperity Gospel (you're poor because you aren't right with the Lord) is its mirror opposite, liberation theology (you're poor because you're oppressed).