What constitutes an RPG?

I think Acer's got the point. An RPG is a game where you play the main character how you wish (instead of an adventure game where you just solve problems so that the almost completely linear plot can continue). I mean, a lot of people consider Diablo an RPG, when I consider it an action game.

I must state, however, that my favourite games are strategy RPGs like FF:Tactics.

Diablo is not an RPG. The character you play is not a specific character to a specific storyline, but yet a character which you specify in a static story line.

Then, would people consider a Sim game as a RPG? If they are just using the reason of "playing the role of something?"

I think by role, they mean role of a character/person. I don't know much about SIMs but if you play the role of one person the maybe

Okay, no. Just simply "playing the role" of a character does not make it an RPG. By that standard, every Sim game on the market is an RPG. In SimCity you play as the mayor of your budding new city and can develop said city in any way you wish. SimCity is not a friggin' RPG.

The three rules I listed are the three core traits that any traditional RPG uses. Like I said, and RPG only has to have one of the three rules.

An RPG is a game where you play the main character how you wish (instead of an adventure game where you just solve problems so that the almost completely linear plot can continue

I don't agree with that in the slightest. How is playing the role of a predetermined character like Cloud from FF7 any different than playing the role of a predetermined character like Link? There's no difference. To define an RPG, you must look at the gameplay, and the three gameplay rules I listed are the foundation of any traditional RPG. Adventure games focus on exploration and puzzle solving, RPGs focus on stat-building, combat strategy and party management.

Since when do you make the rules on what makes a game what?

Look at the name. Role Playing Game. That means a game in which you play the role of a character.

role-playing game
n.
A game in which players assume the roles of characters and act out fantastical adventures, the outcomes of which are partially determined by chance, as by the roll of dice.

Yes, so making Zelda not a RPG as said, cuz everything in the game is static.

I dont know much about FF, but it has random stuff in there? Which makes it a RPG?