With an adaptation of (in this case two books) to a movie you are always going to have to lose stuff. If you put everything in then it would be too long.
The best you can hope for is that the feel of the book is kept and in this case I think that's been achieved. Frankly I don't want to see everything that was in both books - I'll read the book if I do.
The Patrician IS mentioned in the books. He's not named and the description is different from Vetinari - but this was Terry's first Discworld book and Vetinari isn't developed until quite a bit later. Terry has gone on record to say that it is Vetinari in Colour of Magic.
As to Twoflower - there is an assumption that he should be Oriental. Why? This is the Discworld not the Earth. In both Colour of Magic and Light Fantastic there is no physical description of Twoflower that suggests he's Oriental looking.
It's not until we get to Interesting Times that we see a country that sounds like China/Japan on our world but again there are no physical descriptions of the people - Terry rarely describes phyical features.
Actually, the Krullians are described as having black skin and white hair - now that IS something the movie missed out on.
Again, I think the movie kept the feel of the books and they did a good job of the adaptation. It looked and felt like I imagined A-M to be and if anything I think it looked a bit more professional than Hogfather.