The other issue is, you mention mods. Why are mods even relevant? Should a game not allow you to do exactly what you say without mods? Creating your own little adventure? A game should be able to stand on it's own merrits. I don't hate Skyrim. I just find it always a bit strange, when ever people speak very highly about Skyrim, mods are quite often one of the points people bring up.
Yeah, a game should be playable and ideally even good without mods. But I feel like you're arguing for a platonic ideal about what makes a game good. What makes a game good to me is what I experience when I play it. Look at KOTOR 2 and the restoration mod, or Vampire: Bloodlines and the unofficial patch. It's drawing an arbitrary line in the sand to refuse to acknowledge what those do for the game.
But modding for BGS games is a whole other level. Many people on the BGS mod forums are interested primarily in the game as a modding platform. There are modders who put thousands of hours of work into a game while having barely played the game itself. Also, many people who do play the game a lot never complete the main quests. No other game offers the mod tools or mod capabilities that BGS games do, so I think it's fair to call modding a major feature of the platform.
Modding deserves recognition as a feature alongside graphics, gameplay, story, dialog, etc.
(I'd also consider "community" a potentially major feature. When you're evaluating expensive products at home or work, it's often a very good --even essential-- idea to factor the user base into the decision. But I don't want to derail my main point. Just wanted to point that out.)