Quantum wasn't really a BAD movie, but it never EVER felt like a Bond movie, y'know? It was too small of scale, even for the new Bond they'd established in Casino Royale. Sure, the super crazy stuff is gone, but even still, the movie just felt very small. The environmentalist message also seemed strange, but that's fine, and different territory to play with.
And they fell into the trap of just Having Things Happen for the sake of plot. In a spy movie you often have random allegiances and double crosses and changes of sides, but in QoS, it felt more random than usual, and people would just show up and declare they were working with THESE guys now, because the plot needed to go there.
It was an okay spy action movie, but as a Bond movie? A bit of a failure.
Skyfall was MUCH better.
Good action, good story, did all the things you want from a modern Bond movie AND brought in more of the old style stuff too! And even worked in some good character stuff, which is rare in Bond movies.
The acting was solid, Javier Bardiem was a blast and gave as good as Bond. There were plenty of good twists, and it really worked as a celebration to the lengthy Bond franchise. It delivered on all the things I wanted Quantum to give me.
...I have less to say about it, because there is less to complain about. ;)