What I've noticed in my own photography is that, for a dimensional look to an image that gives it real depth, what tends to be required is a biting sharp image where it's in focus, and then a smooth transition into the out of focus areas. Images that're all sharp and then all of a sudden complete bokeh, while pleasant, don't tend to have that same depth (of course, there're exceptions).
Some images of my own as examples...
This shot is super sharp around the couch/his face area, and smoothly transitions into soft background in front and behind that point, letting the in-focus area contrast nicely and pop out well, IMO.
Similar effect here.
In this shot, since the subject is in the background of the image, there's not so much of a 3D pop to it, but the smooth transition into sharp focus gives it a nice sense of depth.
Bit of a boring shot but again - very sharp focus, smooth transition into blur.
I think this is a good example of a shallow DoF, bokeh shot that doesn't really pop. Here, there's no transition - it's either sharp focus or blurry background, so the subject isn't able to stand out as much.
All of the above photos were taken on an APS-C camera. I find that generally, normal to wide perspective lenses are most effective at this effect - fast 50s and 35s (FF equivalent) in particular. I don't like telephotos as much for this reason as they transition too sharply, for the most part, into pure bokeh, and don't "pop" quite as much. There are, of course, exceptions:
Taken from
Steve Huff's D700 review, using a Zeiss 100mm f2.
I hope my images are fairly convincing of APS-C's capabilities of having that dimensional look to photos. I do, however, agree that full-frame makes it a lot easier to achieve.
For example, this shot, taken on APS-C, has a decent amount of pop but on full-frame, I would've been able to stop down a full stop and retain the same depth of field, increasing the sharpness where it's in focus, and giving it that much more depth.