But for anything other than large prints, would you really notice the difference between consumer and pro-grade lenses?
Oh, absolutely.
Well it helps if we don't differentiate between "consumer" and "pro-grade" but merely "good" and "bad."
First things to note are the physical properties of the lenses - focal length and maximum aperture. Lenses with a larger maximum aperture are generally more expensive. They're called "fast glass."
For example, a 50mm f/1.4 lens is a very fast lens. This is because, at f/1.4, a lens allows 16 times more light through than at f/5.6. Since it allows 16 times more light at f/1.4, you can shoot at a lower ISO (for less noise) and faster shutter speed (minimize blur or freeze action).
As an example, you will have the same exposure (total light) at 50mm, f/1.4, ISO 200, 1/120 s shutter as you would at 50mm, f/5.6, ISO 800, 1/30 s. The former shot will have much less noise and, probably, motion or handshake blur than the latter shot.
Most cheaper lenses have smaller maximum apertures. For example, the "kit" lens for the Rebel XT has a focal range of 18-55mm. At 18mm, it has a maximum aperture of f/3.5 and at 55mm it has a maximum aperture of f/5.6. This lens is about $140 or so on the market. By comparison, Canon makes a lens with an almost identical focal range (17-55mm), but has a maximum aperture of f/2.8 at all focal lengths. This lens allows in 4 times more light than the 18-55mm f/3.5-5.6 at 55mm. It's over $1000.
Obviously, you can see how a "better" lens that has a larger maximum aperture can help you get a much better shot, especially in lower light conditions such as indoors.
On top of mere numbers, some lenses are significantly sharper than others and/or have much better color than others. This really does make a difference. A cheap lens will give you crappy, unimpressive pictures. A good lens can allow you to shoot in a significantly wider range of conditions and get obviously sharper, brighter, more vibrant images.