The very least of the changes was to change the British main character to an American to make him seem "more of an outsider". The change in nationality was pointless, but if it had just been that and they had made a genuinely good film which had captured the spirit of the original to an extent, I could have gone along with it. But they did not make a genuinely good film which captured the spirit of the original. Nor did they make a genuinely good film in its own right which didn't capture the spirit of the original. They made one of the worst films I've ever seen. In its own right.
It's not a big deal in the great scheme of things, but I think it was a real missed opportunity to make something special. It was a mistake, in my opinion, for them to adapt a book that they didn't actually like for the big screen.
The series predated Harry Potter by a good few years, but has some things in common with it. For example, the main hero in it is an 11 year old English boy who discovers at that age that he is a wizard (or Old One in the terminology of the series). The series is a story of a prolonged battle by the 'Light', a supernatural order of incognito theoretically immortal beings to prevent their evil counterparts ('the Dark') from rising and taking over the world and the hearts of humanity.
I think it's great. Magic, elements drawn from British mythology (the Arthurian aspects really appealed to me as a child), time travel, and some very mature recurring themes for the intended age groups (notably love and betrayal, and the need to sometimes do terrible things in order to do 'good'). It feels much more 'adult' in tone to me than the Harry Potter books (though I did like them).
There were a few well known actors in the 2007 film.
overview, first billed only:
Alexander Ludwig ... Will Stanton Christopher Eccleston ... The Rider
Ian McShane ... Merriman Lyon
Frances Conroy ... Miss Greythorne James Cosmo ... Dawson Jim Piddock ... Old George
Amelia Warner ... Maggie Barnes
John Benjamin Hickey ... John Stanton
Wendy Crewson ... Mary Stanton
Emma Lockhart ... Gwen Stanton
Drew Tyler Bell