There's this horrible tradeoff in camera lenses between suitability for a given purpose and inconvenience in having to carry and switch out extra glass, some of it quite heavy. I have two lenses for my current camera, a 17-85mm image-stabilized f/4.0-f/5.6 EF-S lens that is suitable for most shots and stays on the body almost all of the time, and a 75-300 image-stabilized f/4.0-f/5.6 EF lens that is useful for portrait and wildlife shots. (Note for 35mm film users: these lenses are being used on an APS-C digital sensor, so multiply the focal length and depth of field distance by 1.6 to get the numbers you might be expecting). The lenses are reasonably light and of decent quality for their price, and cover most of the things I would want to do with a camera. There are two notable exceptions.
The 17-85mm lens has poor quality below about 24mm. Barrel distortion and chromatic aberration can be quite visible, depending on the kind of shot you are taking. When shooting in RAW mode, you can correct for these things subsequently, but it is something of a nuisance. It's best to treat this as a 24-85mm lens with an emergency 17-24 wide-angle mode that you don't generally want to use, and 24mm really isn't that wide a field of view on an APS-C sensor. I could get around this by purchasing the 10-22mm f/3.5-f/4.5 lens, which is a very high quality lens for its price (and I had a chance to play around with one recently, borrowed from a friend), but it lacks image stabilization, and although the price isn't in the 4-digit range of Canon's L-series lenses, it's not exactly pocket change, either, and forces me to carry three lenses instead of two. To a certain extent, I can get around the wide angle problem with perspective compensation and digital stitching, but if I didn't have to carry the extra weight, I'd really like to have the extra capability this lens provides.
The lack of image stabilization on this lens ties in nicely to the next problem: these are all slow lenses. Image stabilization gives you an extra two to three stops to play with in terms of being able to take a handheld shot without causing it to become blurry, but the extra-wide-angle lens is only one stop does absolutely nothing to help the fact that a small aperture gives you a large depth of field, and it's currently not possible for me to take a nice portrait or flower shot with the background completely blurred. This is much, much harder to fix with digital editing. I've found that I like to take portrait shots at 60-90mm when in a small area, and could possibly live with a minimum of 70mm at f/2.8, such as could be found on the 70-200 f/2.8L EF IS lens. The problem with that lens, however (other than its price), is that it's a huge, honkin' piece of glass, almost eight inches long without a hood and weighing in at three and a half pounds. For comparison, my 75-300 lens is only about a pound and a half.
There is hope on the horizon, however, as Canon has started coming out with a Diffractive Optics series of lenses that drastically cut down on the weight, and by making lenses (the EF-S series) specifically for APS-C sensors such as those on the 20D, which again cuts down on the weight. The 70-300 DO IS lens has caught my attention, but unfortunately it's only f/4.5-f/5.6 — worse than my current 75-300 lens
What I am hoping will come out soon is a 70-300 f/2.8 or faster IS lens either in the EF-S series or the DO series, or both combined. At f/2.8, special autofocus mechanisms can be brought into play, and it ought to be enough for portrait shots. An EF-S DO lens would probably be frightfully expensive, but amazingly light, and I'd pay well for a light lens that gave me both IS and good speed at the focal lengths that I use.
