FF gives you massive FoV ( I use a 17-40mm) and much narrower DoF (if the subject is the same size in the view finder for both FF and cropped camera).
There are plenty of ultra-wide angle lenses out there, e. g. Sigma's or Canon's 10-22 or the various 12-24 mm lenses. You are right about the depth of field as that is determined by the focal length. You can get equivalent focal lengths for cropped sensors these days. (The only exception I can think of is a 14 mm FF non-fisheye lens, but you could replace your 17-40 with a 10-22/12-24 lens easily.)
Also, FF are much better (in general) in low light shooting but maybe the new 40D is an exception. On the down side, the /$%/$% sensor gets dirty much faster too :-S
Full-frame cameras are
prohibitively expensive and require
premium glass, i. e. nobody except for a select few can afford FF equipment. Buying glass on a big, big if that in the foreseeable future, full-frame sensors will be offered in the price range of the 40D is pointless, unless you are a pro and have decided
that you will migrate towards a FF body.
Also, full-frame sensors don't have noise levels that are leagues apart from crop sensors, they are marginally better at best. Crop sensors are so good that neither noise nor image quality is an issue anymore for 90+ % of the applications. In particular, the small difference does not IMHO justify the factor of 2 or 2.5 in the price between a 5D and a 40D, for instance. I've taken this snippet from
dpreview's review of the Canon 5D:
dpreview said:
The overall noise performance from the EOS 5D is very good and by our measurements almost identical to the EOS 20D.
dpreview said:
The EOS 5D exhibited slightly lower chroma noise (colored speckles) than both the EOS-1Ds Mark II and EOS 20D, although couldn't match the D2X which has a more monochromatic appearance to noise.
My guesstimate is that Canon has improved the noise levels of the 30D and 40D compared to the 20D, so that noise levels might actually be better than that of the 5D.
I have nothing in principle against FF cameras other than that they aren't affordable and the huge price difference doesn't pay off for most of us.