Interesting observation. Shallow dof 'pop' can be induced by the contrast between very sharp in focus areas and the remaining blurr, or 'bokeh'. So some lenses have really good bokeh; the EF 50/1.4 for instance, but it doesn't display very well against the sharpness and contrast of that 50 mil lens at f1.4. So my initial reaction is that you are not getting proper focus when wide open, either due to the need for AFMA, or a bad copy of the lens.
However what prompted me to reply is the fact that the 24-70/2.8 uses ground glass aspherical elements ( very expensive to produce) and the 35/2 IS uses moulded ( cheapish). Now I have often thought that these moulded elements are getting better and better; the 35/s IS is definitely very good, but some of the lenses that I have, or have had, with the much more expensive to produce elements in them do seem to give, for want of a better description, a 'liquid, 'glassy' quality. I'm not saying that I could pull the difference out in a blind test all the time but I see it in some of my pictures taken with those lenses.