I never met Seth Vidal in real life, we only interacted on IRC and mailing lists, but his effect on me personally is more than simply his software (yum, et. al.), though that by itself is huge. His primary role in my life was as a mentor and example of how to function in a community project. Like the vast majority of contributors, I came to Fedora after Seth did. Seth was welcoming, helping newbies learn the ropes and pointing to places in need of more hands. Seth was amazingly patient, and had an amazing ability to answer questions with grace and tact. Whether the person had found a bug, made a minor mistake, or simply not read beyond the second line of the man page, Seth would answer not simply with boundless knowledge and experience but with kindness.
My thoughts are with your family, Seth, and you will be missed greatly. Hopefully they can take some comfort in knowing that they are not alone, that a great number of people from around the world mourn with them.