Before someone goes down the nonsensical line of debate that "fanboys are dumb, they are always wrong like for Heath Ledger", I would like to remind everyone of a few things.
1) Heath Ledger was an Oscar nominated actor before playing the Joker
2) Examples such as Heath Ledger are the exceptions not the norm
3) You generalize fanboys as cynical douches. I suppose it's convenient for your argument to ignore times when fanboys have been both enthusiastic and right, eg. Emma Stone as Gwen Stacy, Ron Perlman as Hellboy, Molina as Ock....
4) ....And been skeptical and proven right. eg. Nic Cage as Ghost Rider, Jessica Alba as Sue Storm, Arnie as Mr Freeze, Topher Grace as Venom
So, let's move on.
SHAILENE'S LOOKS Yeah, this is the main line of argument everywhere. I, too, have been quite a bit of a prick about it. But the fact is, she's not that bad looking. In fact, she's pretty cute and adorable. But I should not be wanting to cuddle Mary Jane. I should be wanting to whip it out and jerk it off. I do not care if it is the Ultimate version of Mary Jane, nor do most people. Ultimate Spider-Man isn't Spider-Man, it is merely an alternate interpretation of Spider-Man. The main series is still the main series, and the one people have cared about for the past half-century. And isn't the Ultimate universe getting cancelled anyways? But I digress...
If Shailene was getting cast as not-Mary Jane, we wouldn't have a problem. Did anyone complain about the looks of "old hags" Rosemary Harris or Sally Field when they were cast as Aunt May. Nope. And that is because Aunt May isn't supposed to be a hot bombshell. She is supposed to be an elderly woman that expresses a sense of warmth, and both of these actresses were appropriately cast. Hence, no complaints. Which is why the problem isn't so much directly her looks, but the appropriateness of her looks with respect to the character she is hired to play.
Here, we are taking a cuddly little girl and casting here as a smoking hot model. The argument that she is only supposed to be in high school doesn't work since most models are either already models or about to become models when they are in high school. The argument that "Shailene IS objectively hot" does not work, otherwise we wouldn't be having this outcry of people calling her unattractive in the first place. And the Ultimate Universe argument doesn't work because nobody cares about the Ultimate Universe besides hardcore comic book fans. Mary Jane is supposed to have a bit of sass, she is supposed to have a bit of spice, and most importantly she is supposed to be smoking hot. This is evident in her very introduction where the entire running joke was "Peter thinks she's unattractive but she turns out to be this otherworldly goddess looking hottie". Then, the concept of "hot unattainable model falls for nerdy guy" has resonated with fans, and is one of the reasons why the character of Mary Jane is so popular. Is it sexist? Well, more accurately it's perversion. But all that is irrelevant. Mary Jane's popularity is owed to her "unattainable hottie dates nerd" factor. Whether you like this fact or not is completely irrelevant, it is part of who she is and how fans perceive her. Hence, it is important.
And is Shailene sultry enough to portray this part? No. Like I said, she is cute and adorable and pretty and all those sugary things. But we need spice here, which is the opposite of what Shailene offers.
And her age... is irrelevant. What is relevant is the fact that she looks like a mid-teenager. Did any of you see Killer Joe last year? If you did, you'd see what I'm driving at here. If you haven't, there is a character in that film who also looks like a mid teenager (played by an actress who is TWO FULL YEARS OLDER THAN SHAILENE) and is presented as an object of sexual interest to at least two characters in that film. But that film was smart about it. That film knew this idea was creepy, and portrayed it as such. To great effect, might I add. I don't think Webb is looking to explore the implications of pedophilia here. We're about to get a straight portrayal of Andrew Garfield (who is about 30 years old and looks it) romancing Shailene (who is 21 but looks 15)...
Yeah, pretty gross. It's fine if you find her attractive. It is also fine if you are ok with a visual of a 15 year old doing it with a 30 year old. Just know that you are very creepy if you do.
SHAILENE'S ACTING ABILITY She is a decent actress but she is nothing special. There are better looking redhead actresses, who can act much better too. And if you honestly think that no young, beautiful, redhead actress is capable of surpassing Shailene's performance Descendants, you are deluded. This all could be salvaged by charm and charisma. But Shailene doesn't have that enough to make up for the rest. This whole line of discussion just seems like a desperate attempt from a pedophile apologist.
Anyways, Cheers!