To me D&D has a number of connotations, some bad but most good.
- It is primarily fantasy role-playing game. In that sense Spycraft, M&M etc are not D&D despite their mechanical proximity.
- It is differentiated from many other FRPs by a very non-lethal and non-graphic, one can almost say abstract, combat. Both protagonists and their enemies can take many "hits" before they are down and even when they are down they are most likely just cinematically "unconscious" rather then bleeding profusely out of their ruptured spleen while trying to hold in their intestines. The lack of lethality of combat is further reinforced by easily accessible healing magic and also easily accessible "resurrection spells".
- It is further differentiated by the colorful and powerful magic that tends to be reliable and without serious side-effects. Further consequence of this is the unusually high number of "magic items" that characters tend to find and use.
- Another feature that is fairly DnD specific is the variety of monsters encountered. Over the course of their adventuring career DnD characters will fight literally scores of completely different fantastic beasts, whereas in many other FRPs the selection is much more focused.
- On the game mechanical plane, main feature of D&D is that it is heavily class-based game with well defined class/race archetypes. Some variability was introduced over the years but in the end what keeps D&D distinct from a number of other FRPs is just how much most characters fit the basic archetypes.
- Other mechanical/flavor bit that is very much D&D trademark is the use of the full set of polyhedral dice.
There are other smaller points but for me every game that fulfills all of the above is either D&D or a D&D clone. Most of the non D&D games can be distinguished by one or more of the above not holding:
GURPS: No heavy archetypes
RuneQuest: No heavy archetypes, magic is not flashy, most of the enemies are humans, broos, beastmen and dragons-kin.
Rolemaster: Combat is deadly and graphic, magic is (usually) not flashy
Warhammer: Combat is deadly(er), Magic is risky, Enemies are humans, demons and beastmen almost exclusively
- It is primarily fantasy role-playing game. In that sense Spycraft, M&M etc are not D&D despite their mechanical proximity.
- It is differentiated from many other FRPs by a very non-lethal and non-graphic, one can almost say abstract, combat. Both protagonists and their enemies can take many "hits" before they are down and even when they are down they are most likely just cinematically "unconscious" rather then bleeding profusely out of their ruptured spleen while trying to hold in their intestines. The lack of lethality of combat is further reinforced by easily accessible healing magic and also easily accessible "resurrection spells".
- It is further differentiated by the colorful and powerful magic that tends to be reliable and without serious side-effects. Further consequence of this is the unusually high number of "magic items" that characters tend to find and use.
- Another feature that is fairly DnD specific is the variety of monsters encountered. Over the course of their adventuring career DnD characters will fight literally scores of completely different fantastic beasts, whereas in many other FRPs the selection is much more focused.
- On the game mechanical plane, main feature of D&D is that it is heavily class-based game with well defined class/race archetypes. Some variability was introduced over the years but in the end what keeps D&D distinct from a number of other FRPs is just how much most characters fit the basic archetypes.
- Other mechanical/flavor bit that is very much D&D trademark is the use of the full set of polyhedral dice.
There are other smaller points but for me every game that fulfills all of the above is either D&D or a D&D clone. Most of the non D&D games can be distinguished by one or more of the above not holding:
GURPS: No heavy archetypes
RuneQuest: No heavy archetypes, magic is not flashy, most of the enemies are humans, broos, beastmen and dragons-kin.
Rolemaster: Combat is deadly and graphic, magic is (usually) not flashy
Warhammer: Combat is deadly(er), Magic is risky, Enemies are humans, demons and beastmen almost exclusively