Or it could be that Avatar's acting was nothing special. The movie was gorgeous; the acting, ho-hum. The characters were all playing archetypes, at some points it seemed like they were
overplaying archetypes (Sam Worthington seemed to be trying a bit too hard to drive home that he was the "dumb marine" type, and Zoe Saldana switched between "tender alien babe" to "tough alien warrior" far too easily), and there was little in the storyline to draw out a Best Actor/Actress performance.
Reminds me of Titanic, which was also shut out of the Best Actor/Actress awards when everything was over. 12 years later, the acting for Titanic seems laughably bad. I wonder if we'll look the same way at Avatar in 10 years.