the six classic ability scores, classes, levels, AC, spells sorted by power level, alignment (FRPG doesn't need it proper D&D with devils,demons, modrons and devas does), Hit Points.
I would say classes, levels, a big power difference between low and high level (which it seems 4e may be doing away with) and a plethora of monsters and magic items. It probably needs dungeons and dragons, just to justify the name, and some of the distinctively 'D&Dy' monsters such as beholders.
It doesn't have to have any of the above, save the name on the book. It would be quite unusual without the three main adventuring types, or without the same fantasy races, but it certainly would be D&D.