The oxygen concentration and pressure are suitable for humans. It's the high concentration of Carbon Dioxide and Hydrogen Sulfide that cause problems. See Exopack
Carbon Dioxide binds to human red blood cells as part of normal metabolism here on Earth. However, given the low concentration in our atmosphere (<.04%) it poses no problem especially in comparison to our Oxygen concentration (~21%). When Carbon Dioxide concentrations get about 4% (Pandora is 18%) that decreases the amount of Oxygen that binds to blood cells leading to Oxygen deprivation and asphyxiation.
Hydrogen Sulfide is a hemotoxin. It forms an irreversible complex bond with the hemoglobin in red blood cells destroying their ability to bind with and transport Oxygen. This also leads to cellular Oxygen deprivation. The concentration on Pandora is > 1%. The Pandora page states that concentrations greater than 1000 ppm (0.1%) can cause immediate collapse even after a single breath. And we're talking a 1% concentration, 10x greater. I think your point is well taken. If Hydrogen Sulfide really is present at a 1% concentration, it's bye-bye rather quickly. You wouldn't have to wait for asphyxiation due to the excessive Carbon Dioxide. With that said, it's all Oxygen deprivation of one form or another so you could consider it suffocation.
Additional references here's a Hydrogen Sulfide Toxicity Chart & Carbon Dioxide Toxicity Chart