SPOILER!!!:
1. Missing out Maximillian Kohler. This was really annoying, because it made major changes to the plotline as well. By not having Max they eliminated the path from the murder at CERN back to Vatican City and the Camerlengo, as well as not having the 11th hour samaritan and hence having to stick one of the Swiss gaurd in his place. I guess it was just too expensive to hire another actor.
2. The hassassin. In the book the hassassin is of Arabic descent. I can kinda understand why they left this out of the film, because I suppose it *could* be seen as discrimination. They also cut out his kidnap of Vittoria Vetra and the subsequent fight between him and Langdon.
3. Saving Cardinal Baggia. In the book, he is the last cardinal to be killed. Drowned in the fountain of the four rivers by the hassassin. This is rather a large plot change, although in the end it doesn't make that much of a difference.
4. The Illiminati brands. In the book the final brand is the names of the four elements arranged to form an ambigram. In the film it is the papal seal. There doesn't really seem to be a good reason for this change.
5. Galileo's Diagramma. In the book this document is completely destroyed. In the film it is loaned to Langdon as a gift for his services. Again, a useless change IMHO
6. Poor accents. Most of the European characters were played by Americans putting on accents. This really annoys me, because as well as sounding *awful* (and in this film, it's CRINGE WORTHY) it's also denying European actors (and there are plenty) from their chance at the big screen.
There were also a load of more forgiveable discrepancies from the book, such as leaving out the BBC news crew and the Archive scene. As well as a goof in which Vittoria's research partner's corpse being *inside* of the secure area, even though he was murdered for his eye
(retinal scanner)
2. The hassassin. In the book the hassassin is of Arabic descent. I can kinda understand why they left this out of the film, because I suppose it *could* be seen as discrimination. They also cut out his kidnap of Vittoria Vetra and the subsequent fight between him and Langdon.
3. Saving Cardinal Baggia. In the book, he is the last cardinal to be killed. Drowned in the fountain of the four rivers by the hassassin. This is rather a large plot change, although in the end it doesn't make that much of a difference.
4. The Illiminati brands. In the book the final brand is the names of the four elements arranged to form an ambigram. In the film it is the papal seal. There doesn't really seem to be a good reason for this change.
5. Galileo's Diagramma. In the book this document is completely destroyed. In the film it is loaned to Langdon as a gift for his services. Again, a useless change IMHO
6. Poor accents. Most of the European characters were played by Americans putting on accents. This really annoys me, because as well as sounding *awful* (and in this film, it's CRINGE WORTHY) it's also denying European actors (and there are plenty) from their chance at the big screen.
There were also a load of more forgiveable discrepancies from the book, such as leaving out the BBC news crew and the Archive scene. As well as a goof in which Vittoria's research partner's corpse being *inside* of the secure area, even though he was murdered for his eye

I guess for what it is it's not a terrible film, but it certainly isn't true to the book, which is a bit disappointing.