There are basically 2 theories.

1) People who think it should have "of Mars" in the title are so cheesed about it that they think no one will want to go see it because of that omission.

2) Remember that sandy Coliseum scene in Attack of the Clones? The preview makes the film look like a two hour long re-imagining of that scene, devoid of any plot, character motivation or reasonable explanation. It says "Here's a human being simply beating the shit out of a variety of CGI creatures. Now, give us your money."

Are you going to see John Carter? Theatrically? Netflix? Torrent?

For me, unless the word of mouth is phenomenal, it will probably be one of those films I say I'll get around to eventually, then six years after it's been out on DVD, I'll see it in a $3.99 bin, say "Huh, I can't believe I never watched that" and then I'll seriously continue walking by without grabbing a copy.

You can also feel free to be one of those people who, instead of discussing it or theorizing on why it might flop, angrily discusses why box office take is stupid and doesn't matter. That's cool, too.