If one of relational databases paradigms is to be tuple oriented we have the biggest limitation here.
If one could design column oriented db, that would improve performance a lot. Vector operations would perform out of the box, indexing, hashing for simple symbol columns lookups, linked lists behind the scenes as engine.
Memory mapping: dumps in huge chunks in microseconds as well as loading those disk images.
And still have use well understood and standard language (SQL) that multiple vendors support.
Imagine how many tools could be designed for interfacing that thing, because of its simplicity.
Wouldn't it be more robust (and KISS at the same time)?
UPDATE
Thanks to all contributors.
Question has been unjustly closed, though i've found your all answers very informative.