1)
Link - All achievements, old and new. Many of the achievements cannot be unlocked until Bungie releases the new maps, AKA, The Mythic Map Pack.
2) Four ghosts have always been on the level Halo if played on Legendary. You can find them underneath the ramp to the right after you defeat the monitor and go back outside.
3) Pretty much all the information and explanation can be found at www.bungie.net
Link. Just check the weekly updates and news articles on the main page.
4) Three of the new achievements have 0 gamer points attributed to them. However, Bungie said there would be a surprise later down the road to those who have those achievements. Bungie has not stated if there would be anything extra upon earning all the achievements.
5) There are now new playlist ratings. Each individual playlist carries it's own ranking. For example, you could be a General in Team Slayer, and a Commander in Team Doubles.
[Edited by moderator darkish_raven, 9/27/2008 9:05:45 PM][Edited by moderator darkish_raven, 9/30/2008 5:44:55 PM]