'Heirloom' usually refers to something valuable. A wrench may have been passed down through several generations, but I don't think anyone would consider it an heirloom (I could be wrong). So first, I would say, of admirable quality. For a quilt, I think this would mean that there was obviously a great deal of skill and work put into it and it meets a certain standard of excellence -- whether in workmanship, design, or as a representation of an era. It would be an excellent example of the craft, as any heirloom piece would be an excellent example of whatever craft it represented.
Sometimes I see this phrase applied to traditional patterns that are complex and require alot of work, whether complex piecing or applique, and in that case, I think it refers to the traditional aspect of the pattern and that the skill required would make it a potential heirloom.