"Quantum of Solace" presents a vengeful Bond who is also vulnerable at the same time. Daniel Craig plays a tough and ruthless Bond who is just as capable (probably more capable) Bond as all of his predecessors, but his vulnerability lies in the fact that he still cares about certain people and principles. Friendship still means something to him, and he actually considers the consequences of his action. This is drastically different from the standard Bond character that we are all familiar with.
Perhaps that's why Quantum is more satisfying. It's not just a typical action flick, but one with a tortured soul and where the line between good and evil is continuously shifting. Is doing the right thing always the best thing to do?
In addition, all the main characters in this film are all laced with emotional burden that haunts them and motivates them at the same time. Psychological torment is the force that drives everyone, either to their salvation or ultimate doom.
Quantum presents a darker Bond than before, and this adds more dimensions to the suave, Martini drinking spy that we used to know. It's satisfying the franchise has taken a new direction, and the box office proves that the audience appreciates it as well.