First, some resources for the discussion:
House Speaker Boehner (R) in his own words
Senate Majority Leader Reid (D) in his own words
Federal OPM guidelines concerning federal personnel payment during furlough
Americans for Tax Reform describe new Obamacare taxes, mentioned here because some (notably the Medical Devices tax effective this year) are stated reasons for GOP resistance
HHS Secretary Sebelius (D) reported by the Hill -- ACA will open, events planned for opening day, regardless of looming shutdown
Sen. Barrasso (R) reported by the Hill -- ACA exchanges "duct tape and chicken-wire"
NPR discusses what a shutdown means
That's probably enough source to get us started.