Well, I've finally finished reading the book. And there are a lot of things in there that I feel should be included in the new movie which weren't in the original movie. Not that I don't like the original movie, I still think it's great, but I can see why some fans of the book may have been disappointed in it. But I also understand that it was a made for TV movie, and as such has a much greater number of restrictions (not the least of which being time and money).
First, I think the story should be divided into two films, kinda like the original was. They should be theatrical releases, not made for TV, and be roughly 150 minutes each. Like the original, the first should focus mostly on the kids, and their quest to destroy It, while the second film should focus on the adults reuniting for the big rematch.
I think each kid's encounter with Pennywise should be closer to the book (Mike and the giant bird, Richie and the Paul Bunyen statue, Eddie and the leper, etc). They should spend a little more time focusing on Henry Bowers' descent into madness, and his manipulation by Pennywise/It. While I think showing Mike's research into the history of Derry and the appearances of Pennywise might be a bit much, perhaps have him mention them during the reunion dinner. I also think that having the kids chase It deeper into the sewers (and the tunnels beyond) is a good idea. Also, maybe have the reunion dinner come before the adults' encounters with Pennywise like it is in the book. Another thing from the book that I'd like to see in the movie is the fact that It is a pregnant female, and they not only have to kill It, but destroy her eggs too. One thing that's NOT in the book, but that I'd kinda like to see in the movie, is to have a post credits teaser showing that Ben missed one of the eggs and it's about to hatch. Oh, and the blood oath at the end of Part 1, when they were kids. That should be in there too.
Some parts of the novel I feel can certainly be left out or changed. A lot of the stuff about the turtle can be left out. While I liked that the original movie had combined the kids' hunting for It in the haunted house and in the sewers into one scene, I think it should be expanded on. Also, I think they should go down there fully prepared like in the movie, rather than be chased in there with nothing but a book of matches like in the book. I don't think Derry needs to be destroyed at the end either. I don't think Bill should cheat on Audra with Beverly either.
As for the big sex scene in the tunnels? I won't say "include it" or "don't include it". As I said before, it neither makes nor breaks the story. Now that I've read the book, I understand why it was put in. The reasoning might be more difficult to get across through film, and if it's cut should be cut for that reason. But it wouldn't bother me if it were kept in.