My 17-40mm was soft as well. I think it is a 'characteristic' of the lens.
I wouldn't agree with that. First, there's the issue of wide angle zooms in general. True, they're not as sharp in terms of absolute lines per millimeter resolution as Canon's 70-200 f:4 or f:2.8 L zooms, which really are exceptionally good. But they're not that
bad. Then there's the matter of scale; a wide angle zoom photo of a landscape with trees will render individual trees so small on the sensor that you're hitting physical resolution limits, while a focal length of 100 mm is more forgiving. Then there's the diffraction problem; as I understand it, diffraction will harm apparent sharpness more at f:16 with an 18 mm focal length than it will at 100 mm, though I could be wrong.
Canon's 17 – 40 mm L zoom is actually rather highly regarded for sharpness, particularly at apertures like f:8 or so, within the limits imposed by optical reality. It won't compare to a Zeiss 18 mm prime, but it's not "soft". I have the older 16 – 35 mm f:2.8 L zoom, and it's...well, not that bad on a full frame camera like the Eos-1Ds III. Used with meticulous technique on a tripod at an aperture like f:8 (stopped down enough to limit aberrations, not enough to hit the diffraction wall), images are pretty darned sharp centrally. Corners tend to show some color fringing (especially if back-lit) and softness, but I've printed some images up to 20x30" and they're pretty good. Used hand-held at wide open apertures, then sure, it'll get pretty soft.