I wish Canon would make a 24-70 or 24-105 equivalent (in FF 35mm) zoom lens for 1.6x crop that has a small, constant aperture throughout the focal range. It would be something like "EF-S 15-45mm f/2.8 [IS] USM" or "EF-S 15-65mm f/4.0 [IS] USM" (IS being a a nice addition, but not absolutely necessary).
I know about the new, upcoming EF-S 15-85mm f/3.5-5.6 IS USM; it has an ideal focal length range, but the variable aperture makes it less tentalizing for me (I have no doubt it will be welcome by many --> see KenjiS :cool. Even if it was just a constant f/4.0 (I'd prefer 2.8, of course), I would be willing to lose one stop on the wide end to gain three stops on the tele end.
As for the build, somewhere along the 17-55's build is fine with me, although I would certainly appreciate L quality build. Still, the optics are way more important than the build, at least in my case (I take good care of my stuff and don't try too many "stunts" like shooting directly under heavy rain without a cover).
But let me make it clear that by no means do I want or expect L quality on a EF-S lens for cheap. I happily spent ~$1,000 for my 17-55, and I'm willing to dish out in the range of $1,200 - $1,500 for one of the two wished-for lenses I detailed above if they have L, or close to L, quality optics and build, and perhaps IS as well.
Obviously, in the long run I intend to go FF (5D MkIII, maybe?) and both the 24-70L and 25-105L are lenses I want in my kit (I will also consider any updates to those if it happens in the timeframe when I am ready to buy). But at the moment I feel a little limited with the choices of quality lenses offered by Canon for 1.6x crop. Maybe I should look into Sigma or Tamron. I have also considered the 17-40 f/4L, which comes close to the 15-45 range I mentioned aboved.
In the meantime, my two mainstay lenses are the EF-S 17-55mm f/2.8 IS USM and EF 70-200mm f/2.8L IS USM, so I have a coverage gap between 55mm and 70mm. I have the EF 28-105mm f/3.5-4.5 II USM, but I stopped using it after getting the 17-55; the quality just isn't there for me with that lens. I use a very simple solution to deal with the coverage gap, and it's inexpensive: walk up closer to the subject/scene when using the 17-55, or step further back from the subject/scene when using the 70-200.