I posted a similar list last year, and I think it is worth posting some of the books I didn't read in 2010. Now, the actual list is absurdly long, but this is a decent representation of books I would have liked to have read and, for various reasons, never did.
For the sake of keeping this list manageable, I limited it to genre books. If I browsed through listings of LitFic and Nonfiction, well, the list would be absurd.
Iorich, by Steven Brust
Galileo's Dream, by Kim Stanley Robinson
Black Hills, by Dan Simmons
The Sorcerer's House, by Gene Wolfe
Under Heaven, by Guy Gavriel Kay
Kraken, by China Mieville
Leviathan Wept, by Daniel Abraham
Who Fears Death, by Nnedi Okorafor
The Third Bear, by Jeff VanderMeer
Gaslight Dogs, by Karin Lowachee
The Loving Dead, by Amelia Beamer
Swords and Dark Magic, by Jonathan Strahan and Lou Anders
The Waters Rising, by Sheri S Tepper
Passion Play, by Beth Bernobich
The Hundred Thousand Kingdoms, by N. K. Jemisin
The Habitation of the Blessed, by Catherynne M. Valente
I would imagine that had I sampled through these books, my two Best Of lists which I'll post in the coming days would look a lot different.