1. This is not a reboot of the 1990 series. It will be more close to the book than the 1990 miniseries was. Of course, certain scenes from the book have been opted out.
2. It's origins will slightly be unclear. For anyone who was disappointed by the revelation that It was a giant spider in the 1990 film, there was more to that. In the book, it explains that It was a primordial being from beyond the beginning of time who existed in another dimension called the Macroverse. Existing with It in the void is the Turtle. The Turtle is a cosmic entity who is It's natural enemy. One day, the Turtle had a stomach flu and vomited out the universe, which also includes the multiverse that Stephen King's novels take place in.
It somehow knew that humanity was going to arise, so it crashes itself onto Earth in the place where Derry would eventually come into being. Every three decades, It awakens from its slumber to feast on the children of Derry, Maine. An extreme act of violence is what awakens It. While It would eat any sentient creature it could get its mouth on, it goes after children most exclusively because children's imaginations are rich and vivid. It's like applying salt to meet if you will. It takes on any form of whatever you are afraid of, but it exists beyond that. It's true form is beyond human comprehension; beyond the Spider exists a series of continuous, ominous rays of light known only as the Dead Lights. Anyone who looks into these lights are driven to insanity or are killed outright. So, in short, the book is more than your average monster story; there lies in it a tale of cosmic horror which is obviously difficult to fully convey on the screen. Really, say that you were watching the movie, and then one of the characters gets thrown into the Macroverse where they eventually meet the Turtle. Yeah....that'd be confusing if you haven't read the book, and it might even make the film suffer. If the writers decide to try to convey the whole dimensional aspect, that might prevent the second part of the movie from getting released sometime in the near future. However, there will be references to the Turtle in the form of LEGO pieces and other toys.
3. Georgie: In this take on the book, Georgie's death will be played as a mystery. Anyone who had read the book or seen the 1990 miniseries will know that George "Georgie" Denbrough was Bill's younger brother who was given a paper boat to play with after the rain ceases. Unfortunately for Georgie, his boat falls down the storm drain...which makes him susceptible to be manipulated by It, in the form of Pennywise the Dancing Clown. So, basically, Georgie is convinced to make a reach for the boat under the false pretenses that there's an entire carnival down there (and of course, they all float). So, Georgie gets his arm ripped off, and his body's found in the storm drain. In the 2017 film, his death isn't as apparent like in the book since Bill and the Losers Club already concluded that Georgie's murder was It's doing. In the 2017 film, it seems as though they're going to play Georgie's disappearance as a murder mystery.
4. Beverly will most likely get kidnapped during one part of the film. Beverly is the only female member of the Losers club, who has a....complicated relationship with her father. So from the looks of the third trailer for the film, It will abduct her, most likely as a means of luring the other members of the club to Its lair. Now many have complained about this because It never kidnapped any kids in the actual book; rather, it'd kill the kid it's after than kidnap them. The only person that he kidnaps is Bill's wife, but even then, it didn't as much as kidnap her; he just manipulated someone's mind into taking her hostage.
5. As for It itself. While Pennywise is its favorite form, many tend to overlook Pennywise. While Tim Curry's portrayal of the evil clown is memorable, he got his interpretation of Pennywise incorrect, at least in comparison to the book. Part of the reason as to why It loved this form was because it repelled children, as most children I know were terrified of clowns. Besides that, It never actually acts like a stereotypical clown in the book; in total, he made only one joke that the 1990 film exaggerated for comedic effect.
6. The Ritual of Chud.In the original novel, It is defeated the first time because of this ritual. In it, both characters involved would have to bite down on each others' tongue; actually, this wasn't exactly how it plays out. It's a metaphorical way of saying that it's a mind game. It was defeated the first time because Bill's imagination was strong, and he forced It into taking an early hibernation to recharge. After that, the Losers club make a vow saying that they would return to Derry if the string of child murders resume. Well, except for Stanley who killed himself so that he wouldn't have to face It again.
7. Certain characters were adapted out of the 1990 miniseries for obvious reasons. One such character is Patrick Hockstetter. Hockstetetter was a sociopathic boy who was a member of a gang of bullies led by Henry Bowers. Henry himself is generally unpleasant: he's a racist, misogynistic, fat shaming, anti-Semitic bully who is broken out of the mental asylum years later by It so that he could help It exact revenge on the Losers club. Hockstetter was convinced that he was the only real person in the world, and when his parents gave birth to a baby boy, his existence was put into jeopardy. So like any other sane person, Patrick smothered his brother in his sleep. Since then, he took the habit of killing dogs and cats by putting them in a refrigerator he found in the junkyard. While he feared nothing, Patrick was somewhat freaked out by leeches. So, It takes on the form of flying leeches that latch onto Hockstetter, sucking him dry. Because his character was so disturbing, obviously he was written out of the miniseries (which was more tame compared to the book as it was aired on daytime television so they couldn't go all out). However, he may actually appear in the 2017 film because a missing poster had his image on it. Many fans had pointed out that the poster had multiple staples on them implying that many children had disappeared under mysterious circumstances over the years.