I think Relational Theory has a lot to answer for. It has single-handedly taught most CS graduates that there is no such thing as a list or an array :-( - the only valid data structure is the set :-(

Most data structures prior to that were probably based on S-Expressions - but because Relational structures were much simpler ("too simple" in Einsteinian terms) Relational swept the board - which is why I can express in a one line query what SQL takes two pages to express!

Okay, I can only think of Hierarchical and MultiValue, but that's two alternatives to Relational, both based on S-Expression style structures, and I'm sure there are more ...

Cheers,
Wol