I am a recent convert to the 24-70 II. Actually, I am still looking for my "good copy." So I'd agree with the QA/QC issues. But, in addition to IQ, I wanted to add that the 24-70 II also has much faster AF, especially in low light, and I believe is focusing in much less light than the 24-105 on my 5DIII. Both of these are likely to be expected as the 24-70 II enables the f/2.8 AF points, but it is a noticeable improvement.
So, in addition to IQ, I am seeing improvements in AF.