I love Harry Potter, but I have always had issues with the movies - mostly stemming from the fact that they started filming the movies when the book series was still being written, so it wasn't a closed canon, and the way JKR put little hints in books of things that would become important in later books couldn't be picked up, and so were cut out of the movies to then become problematic when they were needed as they were a big plot point in the later movies.
I think that the child stars suffered a little bit from having so many talented and well known British adult actors in the movies, especially the first two - these were kids where the most experienced was Tom Felton (Malfoy) who had done "Anna and the King" and "The Borrowers" before HP and most of the rest had done a few school plays or drama club, acting alongside the likes of Dame Maggie Smith, Richard Harris, Warwick Davies, Robbie Coltrane, Alan Rickman, John Hurt, Julie Walters etc etc (and that's just the first film) and it was always going to be clear that there was a gulf in talent that really wasn't the childrens' fault.
As for the trio, I have always found that Daniel Radcliffe is just a bit ... awkward, and Emma was always trying too hard. I loved Rupert (I still do XD) but he suffered from the fact that in the later films a lot of his lines and character progression was given to other characters so he was left being one of the main trio with very little to actually do beyond the comedic stuff (which he is actually good at, he has great comic timing)
I also felt that the films suffered from having a lot of different directors who wanted different styles and the cast to portray their characters differently whilst also trying to keep things vaguely consistent, this is why Prisoner of Azkaban is my least favourite movie - I love Alfonso Cuaron as a director and he makes amazing films, but as the director of a movie in a series, he's awful and a lot of the things he took out of the movie in order to make room for his stylistic choices affected the future movies and how awkward/rushed things were.