An iconic design is SYMBOLIC: It may have come to represent a period of time, a fashion, a set of ideals, a design principle or a whole design movement.  (Mini Skirt, Casio F-W91, Penny Farthing) 

An iconic design has a rich PROVENANCE: It my have an influential role in history that affected people and perhaps changed society.   (Mobile phone, Charkha) It may have been so effective, useful and trustworthy that it has become ubiquitous (paper clip, Bic Biro, Converse) 

An iconic design has RESONANCE with people: It is remembered fondly and still evokes nostalgia and passion. So much so that it may still be in use now.   (Vespa scooter, Swiss army knife, A-10 Thunderbolt, Barcelona Chair) It will have played a part in people's lives and become part of their life-story, an integral part of personal and social narrative.

An iconic design is UNIQUE: At the moment of its creation would have been nothing like it at all but it was then copied by others, inspired similar products, influenced public taste, defined an aesthetic style, and possibly launched a design movement. (iPad, Fender Stratocaster, William Morris textiles designs)