EGM has impressions of the singleplayer and multiplayer. I won't tell everything but here's some information:
*first mission seems to take place in Kenyan wilderness.
*enemy AI is much better, very smart even on normal.
*brutes won't just berserk all the time, if you knock all armor off they will but they usually stay in the back since they're the leaders.
*brutes have new toys: personal shields, jetpacks, etc.
*enemies are more aggressive and can climb over pretty much anything.
*water dynamically deforms (flows around dead bodies).
*lots of factors into sound effects )supersonic sniper bullets whizz by before banging, soil and rock makesound effects different, etc.)
*seargeny johnson is supposed to look a lot older (graphicsable to show his age this time).
*can use gravity hammer like sword (sends away vehicles and grenades).
*3-4 person human banshee (MAKES ME WET).
*story to be much better, they admit halo 2 didn't work out as they wanted, writing lead says no "raiden" moment (MGS2 reference).
*many marathon references in halo 3 (possible story connection?).
*campaign scoring; points for kills, headshots, taking down shields, sticking grenades, etc. medals to be awarded for good/bad things you do. all stats on bungie.net.
*skulls to return (do odd effects in singleplayer if you find them( and effects stack so you can have all of the skull effects on.
*can change ANYTHING in multiplayer (speed, gravity, invulnerable in certain areas, etc.) with map editor in real-time. don't actually physically alter maps, just move objects and change settings, done from persepective of a monitor (those floating orb-like robots), press up on D-pad to turn monitor into player to test level. you have a spending limit before you can "buy" anymore objects for a level though. can be done simultaneously over xbox LIVE. can have innovative game types such as two teams with a monitor on each team dropping weapons and vehicles for his team. keeps track of score.
*new human vehicle called "elephant": three stories tall, carries about a dozen passengers, slow, can carry a mongoose in belly bay, CAN'T be destroyed by spartan laser (completely indestructible), has turret, can respawn troops, hold home flag, hold return point for enemy flag.
*most awesome news for last: the scarab (that huge crab thing in the second level of halo 2) has AI this time and will chase you down. there are more ways to take it down.
there's a ton of AMAZING screens in the magazine. there's also a chart that shows the estimated scales of the vehicles.