I got my start in Adult and Gaming as most people know.. It's like taking an internship at a really large brokerage firm, and then going off on your own to become a hedge fund manager.
I doubt the adult industry is the same as it was when I was in there, as far as ease of use and cheap traffic/content/marketing is concerned, but although it is one of the most profitable industries around, it's also one of the most cutthroat competitive and coniving ones out there. Mainstream is definitely more wide open, easier, more fun, and a hell of a lot more untapped as far as cash potentials are concerned. You can literally make a living out of launching a new mainstream niche information site every day, for the rest of your life, and you'll still never dominate all of them out there. The earnings potential is huge in the longrun. But I guess that's the beauty of the adult business, it's fast cash for peddling porn is exciting in it's own right, and attracts a lot of the younger male crowd to strike it rich quick.
If you can make it big in adult/gaming, then you can dominate in mainstream. That's the approach I took, and I'm living proof of it.