Mark Waid is a well known comic book writer hailing from the U.S., but more specifically, Alabama. His experience is mostly with DC Comics, with some of his most popular runs being on The Flash and Justice League of America as well as many other series. However, he is known in most circles as the man that scripted the iconic Kingdom Come series, which has gone on to become one of the classic templates for the superhero reconstruction story ever. His most recent work is his own independent series, Irredeemable, focusing on the Plutonian, a Superman-esque hero who out of the blue becomes a sheer psychopath towards his own ilk as well as the rest of the planet. He's also launched a companion series, Incorruptible, which stars a supervillain who decided to turn good after Plutonian went insane. He's also writing an ongoing sequel comic to The Incredibles.
Waid came into his own in the mid-90's, a time when comics were obsessed with making things Darker and Grittier. Waid made his name by bucking this trend, penning light-hearted but emotionally gripping stories which hearkened back to the optimistic tales of the 60's, starting with his acclaimed runs on The Flash and Captain America. He also co-created Impulse, a Flash spinoff which at the time was just about the only humor-based superhero book being published. Its success helped inspire other humorous series such as Young Justice and Deadpool, and helped bring an end to what many fans consider comics' biggest Dork Age. Recently, at Marvel, Waid has been writing Daredevil in a Lighter and Softer series that seeks to reverse the unrelenting Darker and Edgier take on the character that has made up nearly the past 30 years of his history. So far, the results have been well received.
Ironically one of his most infamous (though generally well-regarded) stories is Unthinkable and its following Story Arc, which took the normally more lighthearted Fantastic Four and made it (briefly) Darker and Edgier via having nominal Anti-Villain Doctor Doom cross the Moral Event Horizon in spectacular fashion, kicking about a million puppies on the way. Waid disliked the popular view of Doom as a Noble Demon Jerk with a Heart of Gold and wrote him as Complete Monster with delusions of grandeur. This didn't last beyond his run, but it did give Doom some of his best moments, all of which are still technically canon.
Of course, as his Kingdom Come writing credit suggests, Waid can do dark, as well.
He's known for a tight writing style where characters often speak in hurried sentence fragments in order to emphasize action. He's also famous for knowing nearly every last tiny scrap of DC Universe trivia there is, to the point that DC sometimes holds "stump Mark Waid" trivia challenges at comic conventions.