The line has always been there since the first roleplaying game and the first adventure game.
First roleplaying game was Dungeons and Dragons (Pen and Paper) and so we can see what a roleplaying game is by looking at how it worked.
Then we can see through history what other RPGs share in common with the first and we can define what a RPG is by seeing what all of those games share in common. And no, controlling a character, leveling said character up or do quests are not the only things that make a RPG. Pretty much 99% of games have you controlling a character in some way, today most games have some kind of leveling up and/or quests, but that does not make a RPG, those are elements that were first encountered in roleplaying games, but are not what made that genre being a specific genre.
We also need to deconstruct all of the RPG genres too, because RPG has subgenres:
- cRPG
- Action RPG
- Tactical RPG
- jRPG
Why are these genres also RPGs? Because all RPGs have the same base element:
-The character or characters you roleplay use their own skills, strengths, abilities, weaknesses, and faults to interact with anything in the world. A RPG uses the character to interact with the game world, not the player. That is the fundamental rule of what a RPG is. From P&P to cRPG, Action RPG, Tactical RPG, jRPG, etc, It is always what they all have in common.
Your character(s) have stats and values and those are used in everything (usually using some kind of "dice roll" or RNG), from hitting the enemies to convincing someone that a lie is truth, from unlocking a locked door to sneak past enemies, etc.
People say that what is important in a RPG is good choices and story, a good and reactive world, believable characters, good combat system, action, dialogue, and whatever else people prefer, but that is still not what a RPG is. That is all what makes a good RPG for each of us, not what makes a RPG.
For example World of Darkness RPG system didn't have character levels, characters do not level up. World of Darkness is a RPG and has one of my favorite RPG systems ever (it is the same used in Vampire the Masquerade cRPGs too). So leveling up is not what a RPG is.
For example people say that a RPG needs quests. But quests are just objectives, and pretty much most games have objectives in one way or another. Quests are not what makes a RPG.
Etc.
Those things are not what makes the RPG genre but what enriches it instead.
TL,DR:
What all RPG genres and games have in common since the first one was created is: It's the characters stats and values that are used to interact with everything in the game world, not the player skill.
Sorry for derailing the thread... But I still don't understand why people keep making it sound like it's hard to know what a RPG is when it's the same it always was in any platform (computer/video games and P&P) and in any subgenre (cRPG, Tactical RPG, jRPG, Action RPG, etc)... Character stats are used for everything in the game, the players only decide how the character act while the rest is out of their control.