Originally Posted by The Guard
-Bane hates the rich...because he wasn't born rich.
-Bane, though we're never actually shown this, really wanted to be at Ra's Al Ghul's side, and resents Bruce for not wanting the same thing he did and working against the league.
-And Bane has an unwavering loyalty and love to Talia, and so he does what she wants.
Bane hates rich people because he wasn't born rich. Bane hates Bruce for not wanting to be a villain, for fighting against a villainous organization and is...jealous of him? And Bane does whatever Talia wants because he loves her and is loyal to her.
Mind you, none of this is actually explored.
How is that not a thin character again?
Actually it is explored, but maybe not to the depth of your liking. What more did you want from them exactly? When you boil things down to bullet points, obviously it's going to seem thin. What makes Ra's in BB a 'full' character versus Bane being a 'thin' character in TDKR? In both cases you only get as much information as you need and as is pertinent to the themes that drove Bruce to seek justice.
I personally was a big fan of getting vague pieces of the characters' pasts that you can match with how they handle situations in the film. IMO this is a great example of getting an idea of a character and his motivations without beating it over the head.
And to the comment that Bane 'just IS evil', the answer is simple - he isn't. His moral code is just different than Bruce's. No Nolan character is as simple as that. That's why it's important that Bane, a man born in an environment that had very little chance of turning out 'nice', saves Talia, an innocent, and takes the responsibility of her safety upon himself. He isn't just a terrorist for no reason, he is because he has seen the horrors of those with power using it unjustly (much like Bruce). Keep in mind both Bane and Bruce operate in ways that are considered illegal, but their methods make them either a hero or a villain. Either way, their goal is to clean Gotham's system of corruption, a noble goal in itself.