Are there any voice actors who did not come back for the finale? Are there any new or notable talents who have joined the cast?
The new character James Vega, is voiced by celebrity Freddie Prinze Jr.
It appears that all major voice talent returns except for Mordin Solus, assuming those characters even have a part in your Shepard's story.
Mordin Solus was voiced by Michael Beattie in ME2. I'm not sure who they used to recast in ME3, but the part is not listed on IMDB, despite the character being fully voiced in the game. It has been confirmed to be a different actor though.
The IMDB page listing the cast is located here: http://www.imdb.com/title/tt1839558/fullcredits#cast
Mark Meer voices Mordin in ME3. He was used as a placeholder in expo demos etc whilst they tried to recast Beattie, but they eventually went with Meer.
According to this article, the most notable new voice actors are Freddie Prinze Jr. as James Vega and Jessica Chobot as Diana Allers.
The article also mentions that "It sounds like every actor from the previous games is back, in fact. That includes Mark Meer and Jennifer Hale, the voices of male and female Shepard."