I have a lens with a small but significant looking scratch near the center. But I don't see any difference in the photos. Where does the imperfection show up? How important is it to get the piece of glass replaced? (It's a Panasonic 14-45mm micro 4/3 lens).
Most of the time, the scratch will show up as a small and perhaps even unnoticeable amount of softening in the image. (It will largely be "outvoted" by the vast majority of the light, which is being focused properly.) In high contrast situations, though, or when the light is striking the lens directly, there may be a significant mount of flare washing out the image and reducing overall contrast.
The scratch will probably also make the lens element more fragile, so it will likely to be easier to crack that lens element if the lens is dropped or given a good bump -- that depends on the type of glass.
Fixing it isn't a critical emergency unless it interferes with the kinds of pictures you want to take, but if you plan to keep the lens for a long time, it would be a good idea to get it repaired at some point.
Not as much as you'd think:
Depends on the damage, and the aperture. That's the reason I stopped using UV filters; I'd rather get the best IQ possible and risk damage which can be ignored or fixed if required. Damage to your lens might affect the resale value; by the same token, as a buyer you could pick up a bargain.
Lens work surprisingly well with dirty or scratched front element. Here is a very good article about it, someone deliberately ruined a lens to test this effect.
Probably you'll see problems with lens flare, but the biggest disadvantage is the resale value, it will be more difficult to sell a scratched lens.
One point that hasn't been made is that it's pretty inexpensive to fix the front element of a lens, usually in the realm of $200. It's not a huge concern in general, but it if you are serious, it's worth getting fixed.
A lens can take a surprisingly large amount of damage to it before it will greatly degrade an image. In the event that a favorite irreplaceable filter has gotten a small conchoidal fracture at the edge or a scratch in it, blacken that area with a fine-pointed sharpie pen, removing (rubbing away) any excess ink outside of the scratch or fracture, and this will minimize any contrast-loss that you might get from when it is illuminated by bright light. I always keep a fine-pointed sharpie and a larger black magic-marker in my camera bag. The latter for blackening cyclone-fence wiring or other metal-grids that I might have to shoot through. Blacking any small section of fence or enclosure wire that I have to shoot around/through makes it nearly completely disappear.
The following appears to be the generally received wisdom regarding marks on lens elements. I have observed most of these effects, but there may be additional factors I have missed.
Marks that scatter light are more likely to affect image quality than marks that block light. (Coating of oil vs scratches.)
Light-blocking marks on the rear element are more likely to affect image quality than marks on the front or internal elements.
Light-blocking marks are more likely to be visible when the aperture is stopped down. Small marks would be expected to show up around F11. Larger marks might show up sooner.
Light-blocking marks in the center of an element (at any aperture) are more likely to reduce sharpness or contrast than marks around the periphery.
Light-blocking marks may appear (at any aperture) in images as veiling glare or flare.
Light-blocking marks may be visible (at large apertures) within bokeh balls.
Cleaning marks, often limited to the coating, may not be apparent in images at any aperture.
I would not expect sensor size or film format to change the aperture at which effects are observable. However, the perceived effect may be smaller, relative to frame size, on larger formats. Consider what would happen if a lens were moved from one format to another without changing settings or camera position. Details seen in the smaller format should still be present in the larger format.
Some lens characteristics, like certain types of flare, (reportedly) show up on digital, but not film, because the sensor is reflective or otherwise responds to light somewhat differently. This could affect the visibility of rear-element scratches on images as light reflects between the sensor and rear element.
Some people claim that filling in scratches with ink will reduce flare caused by scratches. I have not seen any positive effect when I've tried it. What I did see is increased visibility of scratches within bokeh balls.