What am I missing? Americana?

Wilco
Tift Merritt
Bethany Dillon

Those are two pretty different artists you mention. Conner Oberst is rather Alt/progressive and Ryan Adams is kind of alt-country-folk-rock combination, and is not nearly as critical and cynical with his take on America. Can you be more specific about what it is you connect between the two?

Based on my own take of those two...hmmm:

Josh Ritter (just introduced to him - brilliant songwriter)
Laurie Anderson's latest, Homeland
Wilco (seconded)
Elliot Smith (kind of down-key)
Jim White (he'll cover the South for you)
Sufijan Stevens (working on those midwest states)
Mark Kozelek (also on Sun Kil Moon & Red House Painters)

More classic takes on "America" that are less like the artist you mention:

Johnny Cash
Leonard Cohen
Dylan

Rodney Crowell
Whiskeytown
Shelby Lynne
Rosanne Cash
Jayhawks
Flatlanders
Jim Lauderdale
Uncle Tuepelo

Americana? So you're looking for music like Ryan Adams? Why dont you load/run Pandora and create a radio station (Ryan Adams Radio). Listen to it everyday for a few weeks & note what it's genre algorithms return and take it from there.

FWIW - ryan adams blows.

If you think the genre Americana is what you are interested in (I consider ryan adams cry-baby-pop) listen to Whiskeytown.

Here's a list of my faves that I dont consider AMERICANA just exceptionally good Rock/Folk music that have been in heavy rotation since their first releases where issued:

Uncle Tupelo (arguably the start of it all [I know Gram Parsons/Flying Burrito Bros - but come on!])
Son Volt/Jay Farrar/Gob Iron
Milton Mapes/Monahans
Collin Herring
Curtis Harvey
Dolorean
Great Lake Swimmers
Elliot Brood
Richmond Fontaine
Richard Buckner
Sera Cahoone
The Vulgar Boatmen
Buffalo Tom
Chad Rex and the Victorstands
Drag The River
Lucero
Drivin-N-Cryin
The Feelies
The Hilltops
J. Tillman
Jeffery Foucault
Kathleen Edwards
Lowlights
Tift Merrit
Wilco

Ryan Adams rules!

Jim Lauderdale's latest, Patchwork River.