Then there's momentum. While Christoph Waltz and Mo'Nique are front runners today, the question is, where will they be three weeks from now? I agree that it's probably Streep vs. Mulligan. Will the veteran who's been nominated way more times than she has won (15 noms, 2 wins, in 1980 and 1983) beat the upstart Brit ingenue?
Also, Up in the Air's George Clooney (who won best supporting actor for Syriana) is not a lock to win best actor: Crazy Heart's Jeff Bridges (four noms, no wins) could sneak up and steal it.
Avatar and Titanic are very different. For one thing, Titanic was a romantic historic epic that earned fourteen nominations and eleven wins. Avatar is unlikely to match that. As a sci-fi (and largely CG) adventure, it will likely earn nine nominations: art direction, cinematography, director, sound effects editing, visual effects, film editing, original score, picture and sound. The noms that Titanic got but Avatar won't are costume, original song, actress, supporting actress and makeup. Avatar won't get any acting nominations at all.
Check out the latest findings on MCN's Gurus 'O Gold, by far the best aggregation of smart Oscar prognosticators (yeah, I'm in there). Must to avoid: Sijmin's Oscar Experiment, which uses a mathematical formula to predict the Oscars: his list of 13 doesn't include Avatar at all.