(I didn't read the link because I try to limit my exposure to fandom-at-large off of my happy little f-list, but what you have reproduced here is an interesting conversation, so thank you for posting!)

I'm not sure Sam will ever fit in well with groups? I just don't think he likes those dynamics as much. It doesn't mean he's shown as being joyless. Off the top of my head, Sam likes dogs, he likes football (Heartache), he likes Harry Potter (D&D Tattoo). I don't think he would enjoy throwing himself into those things wholeheartedly in the way Dean likes slipping into different systems and personas. I think that's character-consistent for both of them. And I like that about Sam? That he's more reserved and not so good at seizing fun where he can. It's a part of why he struggles with the life in a way that Dean doesn't, because those small opportunities to recharge aren't available to him.

Same with the guest characters, though I do think he's connected nicely with more than just Jodi. I think it's a part of his characterization that he prefers to be Library Guy and let Dean do the charming and fast-talking. It's a part of how they compliment each other, that Dean likes socializing even though Sam's the one that is really interested in joining society.

I wouldn't want to lose that part of the character. I don't think it's bad writing, I think it's subtle writing, and I find it more appealing for that. I get that it's tough to watch, because it's so easy to sympathize with Sam's struggling, but to me that's a mark of good writing, that I can understand what he's thinking even if he's not taking up a lot of space in the foreground.

(I am, otoh, extremely frustrated with how some, though far from most, Dean fans seem insistent on reading Sam, but of course that's an unfortunate given.)