Alan Moore is one of the more interesting personalities in the comics industry. He is unbelievably talented but more than a little petulant. He reinvorgated comics in general with his scripts for Swamp Thing back in the '80s, the comic which eventually led to an entire line, Vertigo, being created as a DC imprint. It published Neil Gaiman's Sandman, Preacher by Garth Ennis, and The Invisibles from Grant Morrison, among others.
Alan's present demeanor probably started shortly after WATCHMEN was first published as a graphic novel (it was originally a 12-part monthly series). The contract he signed with DC Comics provided for Moore's full ownership of the characters and book upon the ending of publication. To his chagrin, WATCHMEN is enjoying its 18th printing (at least!) and shows no sign of slowing down. Due to the complexities of work-for-hire contracts, Moore is also not getting his fair share of royalties due him for WATCHMEN merchandise and action figures, something DC executives have stated they've tried to fix over the years -- but are probably being stymied by their parent company, Warner Brothers. Those problems caused Moore to leave DC in disgust after singlehandedly reinventing the medium with sophisticated storytelling techniques and more mature content. He joined Wildstorm (at the time, an Image imprint) and created ABC (America's Best Comics), often portraying thinly veiled copies of DC and Marvel Comics characters. (Ironically, DC bought Wildstorm years later, and Moore found himself working for his old nemesis again, and had to be courted by Wildstorm's president, Jim Lee, to stay).
Moore also has his troubles with Marvel Comics, and has blocked reprints of the English version of Captain Britain that contain his scripts, although rumors persist that this area has been cleared -- still, there are no reprints.
All this and other things paint Moore as the little guy fighting Big Business for rights to his personal creativity and vision. And while that's true to a certain extent, Moore sees nothing wrong in writing "literary ****ography" (his term) and using three characters he DIDN'T create as his main protagonists: Wendy from Peter Pan, Dorothy from the Wizard of Oz, and Alice from Alice in Wonderland, in a graphic novel called "Lost Girls."
Nowadays, a lot of comics fans who originally championed Moore's cause against DC are turning against him. He seems to have a problem with EVERYBODY. No one denies his genius, they're just getting tired of his rants.