Of all the changes from the books they've made, I'd really have to say those two are actually positive and make sense. Luna getting so much screen time is a consequence, I would say, of them cutting so much but not being able to cut many scenes with her, else her inclusion in the Ministry gang would come out of left field.

And Bellatrix, yeah, she's different from in the books, but she's still scary. Ledger's Joker is really going too far for a children's movie/book. I think Bellatrix actually acts more insane in the movie - where she has some scary moments as well as the giggling and pouting - than in the book where she's mostly a straightforward homocidal maniac and only occasionally has the cackling.