I grew up looking at it, came very close to losing two family members in the WTC (my FDNY uncle, and my aunt who worked in it), and know a few people who died in it (my uncle's firehouse). It isn't too controversial.
I'll take an educated guess that the trade center feature/flaw was the exo-skeleton supported design. The heating of the inner floor web support beams effectively allowed them to bend and buckle the exo-skeleton inwards, contributing to the ultimate collapse of the most damaged floors and the upper floors propagated futher collapse through the outright force of their momentum.
In a nutshell, had the towers been constructed with a relatively small number of inner support columns, they probably would not have collapsed.