What's brilliant about Derren is he's elevated magic and mentalism above the mere display of effects, into something truly personal, intimate, challenging, and dramatic. And, in a lot of cases, he's done this with incredibly basic effects which anyone can learn. I would say that the most important lesson to take away from his success, is that the tricks are secondary to the character you create. He's not "the guy who does [insert trick name]", in the way that some magicians might be "the guy who produces doves", or "the guy who eats lightbulbs", he's just "Derren Brown" a man who's own persona is the draw.