1. I once saw a fan state that Dean, unlike Sam, doesn't express his anger at his brother. That's why Dean carries so much resentment over the seasons, the argument went, while Sam does not.
(Bear in mind that I haven't seen past S10, and the meta I saw was written before that time.)
The #1 problem with this statement is that it rests on the idea that not only is physical violence not an expression of anger, but that walking away from a situation, or withdrawing emotionally from someone you are mad at is a more intense expression of emotion. Just to recap: Dean punches Sam in Bloodlust, twice in Metamorphosis (while also threatening Sam's ally with death and trashing the motel room during the course of his argument with Sam), beats soulless bloody and unconscious in You Can't Handle the Truth, and punches Sam to the ground in The Girl Next Door. Yet somehow all this doesn't qualify as Dean expressing anger about Sam's actions?
How does sexism factor in? Imagine if a female character behaved like Dean did. Imagine a woman who punches people when she gets really mad. Now imagine someone trying to claim that this woman doesn't express her anger. They would be laughed off the internet. Since Dean is a man, his sporadic attempts at roping in his anger and being super-forgiving of Sam mean he doesn't fully express his true emotions. No woman who acted like Dean would be granted that leeway.
2. When I was more active in Tumblr-based SPN fandom, I encountered a lot of Dean fans who were unhappy with how Cas and Sam treated him. A lot of them disliked how pushy Sam was in getting Dean to open up to him in early S2 and S4 (oddly, early S7 doesn't get mentioned much…probably because what Dean was keeping from Sam was something Sam had a right to know). One fan brought Lisa up as an example of a character who treated Dean well but was also firm about her own boundaries.
I, to put it mildly, disagree. Lisa undoubtedly is empathetic and emotionally supportive of Dean, but she is in no way protective of her own boundaries, and in no way is comfortable standing up to Dean. Just going over the episode transcripts: in 6.01, Dean lies to Lisa, claiming he was setting up a poker game when he was investigating a possible supernatural problem. Despite the iffiness of his story, Lisa accepts it with an "Okay". There's no indication that she's unhappy that her partner is probably lying to her. Later, when Lisa finally decides to call Dean on his lie, she's not mad at all. She's low-key. "It happens," she says when Dean admits he might have gotten worked up over nothing. Lisa asks for no further information, even though Dean is concerned enough about the situation that he asks her and Ben to leave the area. Throughout the entire episode, Lisa passively follows Dean's lead, not asking questions. The only time she stands up to him is when he apologizes too much to her - and she stands up to him by informing him that the year they've spent together (after he showed up unexpectedly on her doorstep and she took him completely into her life) was the best year of her life.
In 6.02, we see more of Lisa being endlessly supportive to Dean and Ben, not even getting mad when Dean forgets they'd planned on going out and orders pizza without consulting her. Later in the episode, after Dean yells at Ben, Lisa finally expresses unhappiness about Dean's behavior. She does this not by speaking her mind, but by carefully asking Dean: "You want to talk about this?" One wonders how the conversation would have gone if Dean had said "No".
Fortunately for Lisa, Dean doesn't say "No". During the course of this conversation, Lisa is eager to assure Dean that she is "not arguing[…]just asking". It's only after all these reassurances that she finally asks Dean what is going on, and talks about her needs and Ben's needs, and her concerns about Dean's behavior.
Later in the episode (after asking if she can be honest), Lisa tells Dean she wants him to leave her and Ben part of time to go hunting with Sam, and that she will be 100% supportive. Never mind that she and Ben are supposedly in greater danger with Dean not around. Never mind that one of the most important things that Dean was providing Lisa was the co-parenting of Ben - something he can't do if he's off getting killed by monsters. Nope, Lisa is endlessly supportive.
I am not the first person to make these points about Lisa. giandujakiss wrote about this back when the episodes were first airing. In this post, she said, "[Lisa is] almost completely deferential to Dean - barely speaks above careful, considerate whisper. Her entire emotional life revolves around Dean.[…]She has absolutely no apparent flaws or characterization of her own, other than 'perfectly understanding, mildly assertive only when Dean indicates he's willing to accept it.' She walks on eggshells around him at all times."
I actually really like the idea of Lisa. I like the idea of Dean dating a civilian, a single mother with a child who isn't interested in the hunting life. I like the idea of Dean trying to balance hunting with domestic life.
But in practice, Lisa, endlessly patient, endlessly supportive, never willing to openly challenge Dean or rub him the wrong way? Count me out. I get the vibe, watching those early S6 episodes, that the writers feared if Lisa acted like a human being and didn't prioritize Dean's emotional life at all cost to herself, fans would dislike her. Judging by the fans who deem Lisa as one of the very few people in Dean's life who treated him well, even arguing that she did so without sacrificing her own emotional needs, this would be true.
These Dean fans resent the way Sam treats Dean. As a male character, Sam is allowed to be human. He's allowed to say the wrong thing, put himself first, and not be endlessly supportive of someone else's emotional life. Lisa, as a female love interest, is not allowed to be any of those things. Therefore, she's one of the few people whose treatment of Dean is deemed acceptable, because she treats his emotional life as more valuable than her own.
In You Can't Handle the Truth, Lisa finally breaks up with Dean. To be honest, I felt her switch from 100% supportive of Dean hunting with Sam to extremely hostile towards the idea to confusing. What changed her mind? Especially given her claim that she felt this way all along? I can fanwank it, but it comes out of nowhere. Obviously, Lisa's characterization was not a priority to the writers.
The break-up is an example of Lisa standing up for herself without Dean's permission, and prioritizing her needs over Dean's, but even as such it's very limited. It only comes after Dean coming home unexpectedly, acting weird, and shoving Ben, and then refuses to explain himself (Note that Lisa only ever seems to try to stand up to Dean when he hurts Ben. Caring about your child is fantastic, but the writing never seems to realize Lisa herself is valuable, too.). After all this, Lisa insists on having a conversation with Dean when he clearly isn't able to. This is the first time Lisa has been anything but submissive or blandly supportive in her interactions with Dean. Then Lisa goes on her truth-telling rant, inspired by supernatural influence. "That came out so much harsher than I meant," she says immediately after.
But how bad was her rant, exactly? Her words are harsh (a harshness she only expressed because the supernatural made her do it), but what she's saying isn't all that critical of Dean. Her main criticism is of Dean's relationship with Sam, and her focus is on Dean's emotional life, not his effect on her emotional life. "You've got so much buried in there, and you push it down, and you push it down." Dean is the one who will never be happy as long as Sam is in his life. She doesn't talk about her emotions about the situation, her internal life, at all. It's all about Dean.
So yeah, the idea that Lisa was supportive of Dean while setting her own boundaries is inaccurate and sexist. Part of setting your own boundaries is being imperfect, impatient, and thinking of yourself first. The show does not allow Lisa to do any of these things: She consistently prioritizes Dean's emotions and perspective.