Taken King's level cap is 40.
They're completely redoing the level system
http://www.gameinformer.com/b/news/archive/2015/08/04/destiny-the-taken-king-redesigns-leveling-from-one-to-forty-4682.aspx
They're also replacing all of Peter Dinklage's lines with Nolan North, who will be voicing the Ghost in Taken King, and re-recording all the ghost lines for Vanilla.
Year One Legendaries staying behind as well (for upgrading, I assume, you'll still be able to use them)
http://www.gameinformer.com/b/news/archive/2015/08/04/destinys-year-one-legendary-weapons-staying-behind-in-year-two-9432.aspx
They're completely redoing the level system
http://www.gameinformer.com/b/news/archive/2015/08/04/destiny-the-taken-king-redesigns-leveling-from-one-to-forty-4682.aspx
They're also replacing all of Peter Dinklage's lines with Nolan North, who will be voicing the Ghost in Taken King, and re-recording all the ghost lines for Vanilla.
Year One Legendaries staying behind as well (for upgrading, I assume, you'll still be able to use them)
http://www.gameinformer.com/b/news/archive/2015/08/04/destinys-year-one-legendary-weapons-staying-behind-in-year-two-9432.aspx