Well, this really sucks. Kim Thompson was a huge figure in comics writing, editing, and publishing — maybe not as well-known as Stan Lee or Alan Moore, but just as significant. Along with Gary Groth, he was the cofounder and coeditor of the legendary Comics Journal in the '70s, and later helped launched Fantagraphics Books, the amazing publisher of such prestigious alternative comics as Los Bros Hernandez's Love & Rockets, Peter Bagge's Hate, and Daniel Clowes' Eightball and Ghost World, as well as high quality reprints of classic American and foreign cartoonists. Thompson and Groth were among the most instrumental voices in arguing for the artistic and cultural worth of comics in the last thirty years, and their contributions as journalists and curators are enormous — it's largely because of them that you see folks like Art Spiegelman and Chris Ware reviewed in mainstream publications. They also broke the story about Jack Kirby's struggles to regain his art from Marvel, which arguably helped kick off the creator-owned comics movement.
The Comics Journal was a huge part of my adolescence, and went a long way to shaping how I consumed and understood both high and low art. With Thompson's death, that really feels like a long time ago.