In Confidential, Billie Piper said she had trouble getting Rose's accent back after doing Secret Diary of a Call Girl; she watched a bunch of DVDs to try and remember how Rose talked, but that's the basic reason why she talked differently (it's not her natural accent).
The teeth? Yeah, I agree they looked weird; no idea what that was about. They didn't look like that in the Confidential interview.
The trailer for the last two eps looks amazing: the Doctor, Rose, Donna, Martha, Jack, Gwen, Ianto, Sarah Jane, Davros, a the Judoon, a Red Dalek, UNIT...
It could be amazing, it could be really bad. As long as RTD doesn't have someone glow and do magic again...