Neonchameleon
Legend
Personally, and this is probably because I consider the game the mechanics and not the fluff, I see tons of things all versions of D&D but 4e have in common. The Vancian wizard was pretty darn close all the way through. AEDU was a huge departure.
Whereas to me there is a vast difference between the 1e "No automatic spells after first level" and the 3E "Two free spells.level".
The simple fighter as an option was possible all the way through
It turned up in Essentials, but this is actual gameplay and one I'm happy to concede.
They turned the entire healing system upside down. Surges, martial healing, magic that couldn't heal beyond your surges, and so forth.
The entire healing system had been shattered by 3E and 4E rebuilt it. The Wand of Cure Light Wounds being readily available was a complete gamechanger.
The healing system for the most part had not changed prior to 4e. The quantity of healing perhaps but not the fundamental system.
The idea that you could go into every fight at full HP (and indeed not to do so was simply bad logistics after about second level) wasn't a change?
So those things are what I consider a games identity. The actual mechanical structures. I realize some of you have some other viewpoint on what makes D&D D&D. For me the rules are always what make a game.
To me how the structures work are more important than that they look vaguely like each other.