I mean, I agree they should've just cast Zendaya as Mary Jane and not do this whole Michelle/MJ fakeout. But does that really make or break the movie for you people? I would rather they'd go full Mary Jane, but the character being well-written is more important than this for me. And it's early to be sure, but reactions for the film seem really positive thus far - yes, I don't put much stock on random people on Twitter, but least that Daniel guy seems very picky and then there's Steve from Collider saying he's hearing the movie is good/great. We should know better tomorrow, anyway.
Everybody can like/dislike whatever they want, obviously, but I feel what's happening here is very much akin to the fan reactions to Iron Man 3 - which I believe is a great movie by one of the best writer/directors to work on a superhero movie. I'd say the Mandarin reveal is brilliant, but no, it's not like the comics. On Homecoming's case, there's nothing as big as that as far as we know, but some fans seem really adamant on hating the movie even before seeing it for some small changes - because, well, they don't seem very important to the actual plot. Spidey is my favorite superhero and I'd love to see Mary Jane, Gwen, Harry and Flash as close to the comics as possible, but the fact that we are getting something different doesn't mean we are getting something bad. It sure lucks fun to me.
Just my two cents.