After some weeks of intense work and real life tasks, I've got two weeks ahead of taking things with calm and try to rest (or at least spend my free time doing mostly non-development).
Apart from finally being able to test my painting lamp and actually painting a bit (nothing major, I want first to finish some half-painted miniatures and scenery props), I decided to go and buy both Fable II DLCs to extend the game's lifetime a bit before starting it again creating an evil alignment character.
And while playing yesterday and today for a few hours, I remembered why I enjoy Fable II so much (and other games like The Witcher, Fallout 3 or the Final Fantasy series) and why I haven't finished yet Neverwinter Nights (either part, but I prefer the first one).
The answer is simple: Because they tell you a tale, a story.
The argument of Neverwinter Nights its a bit typical, not engaging enough to make me desire to progress. I usually park the game few days after playing it because all quests are very similar and just feel "plugged in". I don't feel like living as my "drow", I feel playing a computer version of DnD.
When I play Fallout 3, I feel like really living in a post-apocalyptic future. I feel like exploring a huge world, discovering buildings, ruined citys, doing small quests... really inside of the game.
And while Fable II offers no challenge (it is way too easy) and is not so big and open as Fallout 3, whenever I play it I feel inmersed into the fairy tales it transmits. The world is so beautiful, the people react to my actions, the bards sing my adventures...
Many people dislike the Final Fantasy series for having too much combat. But combat in FF, in the end, is just a way to slow the pace, to extend the time you need to reach the end of the game and so finish the story it tells. I don't care having to level characters on purpose (instead of just as I go through the game), because the stories the games tell are engaging.
So, for me, the single most important factor or aspect of a computer RPG is not how well it implements a given set of rules, or how easy or hard are the battles, but the presence of a good story and a good sense of inmersion around it.