Description of problem:
Many if not most cases of bad downloads are due to simple truncation. Browsers suck at downloading, and if the server has a hiccup, can just silently truncate the download. Command-line tools like wget are much better but many people don't use them. If checkisomd5 checked the size of the actual image compared to the ISO header size, it could give a specific error message if the image was too small. (Note that Fedora ISOs are hybrid and are bigger than the ISO header size, however it's my understanding that the mediacheck ignores anything past that size anyway.)
If the install type allows checking the ISO size quickly without reading the entire image (for example a hard drive install), this check could be done by anaconda without involving the mediacheck, so even someone who didn't do the mediacheck would be warned. I don't know if that's practical, and if it is it would be a separate RFE.
Version-Release number of selected component (if applicable):