Maus being "unimpressive" boggles my mind a bit.
Since you've gotten a lot of recommendations in the dark indie-superhero/post-superhero vein, here are a few more "lit comics" (for lack of a better term) recommendations:
-Maus by Art Spiegelman, of course.
-Jimmy Corrigan: The Smartest Kid on Earth by Chris Ware, my personal candidate for greatest comic ever made. It's the story of a nerdy milquetoast of a guy struggling with the imprint that family has left on him and meeting his estranged father for the first time. The style is radically experimental, and the whole package really *works*; the form is impeccable, and the content is layered, complex and really breathtaking.
-Acme Novelty Library #20 by Chris Ware, which is the sad and conflicted life of a rich heir and business shark contained in one beautiful, lushly hard-bound book; he's born on the first page and dies on the last.
-Quimby The Mouse by Chris Ware, if you think you can stomach some of his more experimental and obtuse stuff.
-Asterios Polyp by David Mazzuccelli, which is the story of the conflict and union of dualistic and non-dualistic thinking told through the personal crisis of an architect who has become respected for his theoretical work but has never had a building built. A formal masterpiece.
-Scott Pilgrim by Bryan Lee O'Malley you've probably heard about at least due to the movie, but Hollywood aside, it's an incredibly funny urban geekfest of a series and imminently worth reading.
-Phoenix by Osamu Tezuka is the magnum opus of the greatest manga artist of all time, the godfather of manga and anime. It's a huge, serious epic, spanning thousands of years. Incredible formally, incredible artistically, just incredible all around.
-Gray Horses by Hope Larson, although really, any of her works will do. Small, quiet, poetic pieces,
-Artichoke Tales by Megan Kelso. Cute, gentle, brutal and meaningful exploration of war, strife and human relationships.
Also, of you like Transmetropolitan, Warren Ellis (the writer) is currently in triumphant stride on his excellent post-apocalyptic Webcomic, FreakAngels, with the inimitable Paul Duffield. Very worth reading from the beginning.