Pre-loading of Halo 5: Guardians has begun so you can now start the long download if you happen to have pre-ordered Halo 5. 343 Industries also has the entire achievement list out for you to peruse.
Also, they've released the entire list of achievements in game. There are a total of 65 with the possibility of adding 1000 to your gamerscore, with the possibility of them adding more after the game releases. My favorite is the last on the list; "Benchwarmer - Spectate a game". That might not be achievable for everyone. The rest seem to mostly be fairly easy to achieve, such as completing the single player missions and opening a gold REQ pack, not necessarily groundbreaking in the difficulty department.