namely, much of the stuff regarding Bev. There's the insertion/playing up of the love triangle between Bill, Bev, and Ben; her flirting with the pharmacist to cause a distraction; and the whole climax being instigated by her getting kidnapped by Pennywise, and having to be rescued and awakened by a kiss (instead of the entire Loser's Club venturing into the sewers together, with Bev as the one they've all designated as being the best one equipped to take on Pennywise with their silver bullets+slingshot plan)

You add all that stuff up, and it's... pretty fucking weird, and kinda shitty that so much was changed in order to sexualize an underage girl. It makes the movie feel a bit flattened out, and weirdly retrograde, like they needed to add a couple stock Hollywood plots into the mix.

The final script for the movie was based on the original script, written by Cary Fukanaga (who was also originally attached to direct), and that original script got leaked online shortly after he left, and guess what: all those changes listed above were all added later on. There were certainly quite a few things that were odd/improved by the final script, but I still can't help but feel a little miffed that the weird Bev stuff was added in, and some of the more interesting stuff was dropped. I can't help but wonder if the reason some of those interesting aspects were dropped in order to make room for the love triangle.

... or maybe they were deemed too controversial. For instance, Mike is barely a character in the movie, but in that original script they include quite a lot of backstory, specifically relating to his dad, who was at the nightclub that was firebombed 27 years prior to the movie; the original script also makes it clear it was a racially motivated attack, where the movie only lightly implies. That reluctance to engage with Derry's racism has me leery about Chapter Two: the incident that reawakens Pennywise in the present is a gay bashing, which was inspired by a real life gay bashing in the 80s that took place in Maine. Stephen King used a fictionalized version of a horrific, real life homophobic murder as the latest in of a series of horrific tragedies that "summons" Pennywise from Its slumber, powered by the hatred and bigotry that exists in humanity. If this big budget, major studio movie is reluctant to even explictly reference a racist firebombing, I can't imagine they'd want to tackle something like a gay bashing.