Tychoxi: So Star Control 2 is an RPG, Half-Life is an RPG, Rsident Evil is an RPG.... they all have C&C in regards to the story and provide interaction and reactivity in the game world based on your choices.
Fenixp: All right, I'll have to change that definition a little so you can see what I see as 'reactivity with the world.' An RPG is a game that lets you play a role of your choosing. Witcher is an RPG, you can make a lot of decisions, from trivial ones like order in which you explore the world, to crucial ones that change the world itself. TES games are RPGs, obviously. Fallout games are RPGs, for the same reasons. Half-Life ain't an RPG, neither is Resident Evil, for they don't allow you to play your role, they allow you to live a story of someone else. And what on earth is C&C? (for that matter, no, they don't have any in-game world to speak of, only gameplay influencing architecture.)
N0x0ss: While J-RPGs are more restrective, it retain the same concept.
You control a character or bunch of character with distinct personalities, the story evolves around them and take into consideration their relationship.
So a "true" RPG for me is a game where you control and develop character/s with a freedom to a certain extinct, but also with important personalities.
Fenixp: I for one think that jRPGs only have RPG stamp because they have stats. Now don't get me wrong, I love RPGs, but the name itself implies the importantce of role-playing. In jRPGs, you almost never get that chance.
Your choices in Star Control 2 make and unmake alliances, help you complete quests, doom or save spacefleets, put you in certain death, etc. And the races react to your choices, if you decide to sell crewmembers to slavery for example, humans will chastise you and getting crewmembers will get more expensive. In HL you make a choice at the end that has storyline consequences: you choose defiance and die, or choose compliance and are put in stasis. It also has reactivity, for example when you overheat the meal in the microwave oven it explodes and someone admonishes you for it. And there's more. In Resident Evil, your choices shape the storyline you play through (characters can live or die at different instances, sacrifice their lives for you, you may avert whole fight sequences, etc) and again characters will react to certain things you do or not do.
Some people will argue the Witcher doesn't let you play a role, it just casts you as a predefined character and you choices are limited due to that, in essence it only allows you to "live the story of someone else".
(C&C = choices and consequences)