These threads are pointless, since 99% of the gaming public and at least 70% of this forum don't understand what an RPG truly is... Just see the various "best of" lists codexers have voted for over the years for proof. In a forum where action/3d metroidvania games like Dark Souls are considered "rpgs", there is nothing to talk about...
The problem with the RPG definition and its misconception is that many elements that are required for a game to be defined as an RPG are shared with other genres. For example C&C can be a part of point & click adventure games. Character stat development can be a part of a game like Assassin's Creed. Most games have a story and dialogues these days, etc. So people tend to mistake a game with RPG elements to a true RPG, and just call everything RPG. Even Super Mario is an RPG these days...
So, to answer your question, for me the most important thing for an RPG is to have a meaningful stat system with skills that matter no matter what stat development you choose, and the ability to influence the world differently (not just combat) based on the character you build. Ideally it should be turn based and entirely stat-dependent. That is what a true RPG, you create an avatar to explore an imaginary world, and control his decisions but not his actions, that is, your avatar is an extension of your will, not of your hands... Essentially, RPGs should be strategy games on a smaller scale, instead of controlling armies or platoons you control a person or a small party of persons... You get to decide what they do at every single time but the success or failure depends on their stats. That is what an RPG truly is.
And that is what is mostly important, and what is mostly lost in modern times.At best we get stellar Action-RPG hybrids, games like Witcher 3 which while introcuding player-skill into the fantasy world, at least they still have good overall quality and stats do matter somewhat, at worse we just get garbage like FO4. We rarely if ever get true RPGs these days.