I think that one of the major factors of the various editions of D&D and what they mean too me are largely effected by what they seam too simulate. I would say that the older the edition the closer it was too simulating playing soldiers in some way. You played a man-at-arms who raided a dungeon.
Lots of flavor was put towards things like taxes and wasting money so that you would need to raid someone too get more money. Which also gave you xp or power. Then each edition after that it went from this low fantasy feel closer and closer too a high fantasy feel.
By 3.5 there seams too be indications that high level people are rather abundant and whole economies could be based off of wishes. This seams too be in contrast too some of its world simulation tools I have used when playing 3.5 Such as rolling up cities and being able too make money in town using profession and crafting.
4th edition of course went into a totally different direction.
All of these are called RPG's but too me each one hasn't entirely emphasized Role playing at all. The rules reflect mostly combat but then say you can role-play. I also feel that skill systems like in 3.5, rune quest and even rifts can take away from roleplaying as you have a mechanical way too deal with the character.
I can't wait too play The Secret Fire and see if it really makes me feel like its a role playing game and not a simulation of battle or fantasy magic.
((Note: I have used all editions and successfully role played, but I have also played all editions on a completely hack and slash level))