These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Merida herself has become this for a lot of people. Either she's a very believable character with plausible flaws and great Character Development, becoming a wiser person in the process, or a whiny, unsympathetic, selfish and yet another cookie-cutter tomboy princess.
Some people dislike the Witch because they feel she's either not funny, doesn't fit into the rest of the movie (and not in the good way), and wastes a lot of screen time since Merida and Elinor have to go back and forth between her place and the castle. Others found her a one-scene wonder.
When the film was released, reception from both critics and the audience themselves has turned Brave into one of Pixar's most divisive movies yet, similar to the first Cars. And since Brave was following on the heels of the poorly-received sequel to that movie, unsatisfied critics are fearful that Pixar's Glory Days are coming — or have come — to an end.
A shockingly large community of fans have taken to pairing Merida with Hiccup, which is interesting due to the fact that Vikings are the (historically) sworn enemies of Merida's kingdom. This one even has been named "Mericcup."
The triplets, crafty little goofballs that they are.
Fergus gets a lot of fandom love for being one of the film's most entertaining characters, and for how much he adores his wife and kids. Also, he's voiced by Billy Connolly, which gets him like a million extra points.
Shy and adorable Young MacGuffin is the favorite pick for suitor, partially because he was the pick before it was decided Merida wouldn't choose, and partially because he doesn't have obvious personality or appearance flaws (which is probably why he was the one Merida was going to pick).
Launcher of a Thousand Ships: Merida has become a victim of this. Fans all over the internet are either making up some Crossover Ship for her, pairing her with their OC, or pairing her up with one of the suitors she outright rejected in the movie.
A large chunk of the movie's Disney Princess merchandise has Merida in the dress her mom forced her in and practically suffocated in.
In a slightly different sense this is also what happened to the actual marketing, which went out of its way to paint Brave as an epic fantasy adventure when in truth, it's a character driven classically styled fairy tale.
Rewatch Bonus: A small one, but keep an eye on Elinor as the suitors are introduced. For all her talk about upholding tradition and her insistence on Merida marrying one of the three, you can can see she's having at least some reservations along the lines of "this...is not what I was picturing as a man to marry my daughter." Could count as a touch of foreshadowing.
Visual Effects of Awesome: Holy CRAP, just look at Merida's hair. IT'S AMAZING!! The movie in general, when it forces PIXAR to rewrite their animation software, you know the end result has to be nothing short of gorgeous.
You can't help but feel bad for Elinor when she gets turned into a bear. Her confusion, her terror, and her struggling to keep herself together is rather painful to watch. It just gets worse once she starts losing her humanity and actually starts lashing out as a full-fledged bear.
Jerkass Woobie: Merida. Yes, her actions were pretty darn selfish, but the fact that she recognizes this and becomes more and more fearful over the fate of her mother makes it easier to feel sorry for her. Also, while her mother IS trying to train Merida in how to handle the responsibility of ruling a kingdom, Merida's frustration with her situation becomes understandable when Elinor tells her that "a princess strives for...perfection." Merida knows that she is far from being perfect, so dealing with constantly being corrected must be stressful, not to mention rather disheartening.