Same gameplay of the old campaign of CE, you can switch between old graphics and new graphics. You get 7 CE Maps put into Reach online multiplayer. all of this for $40. You are paying $40 for a game that is 10 years old.
-technically two games in one. old engine and Reach engine. -all core CE maps with using Reach's online mode. (big mistake, its a horrible COD knock off. Halo 3's is better.). -(rumor) three of the six exclusive maps found only on the PC version to be included. -(rumor) additional bonus campaign mission. -a separate download code to play these maps using Reach disc. -an unknown Firefight map (rumored to feature the Flood as the enemy). -Kinect support (kinda eh). -co-op mode over XBL -Terminals
Its 3 Engines actually. People need to stop using Reach's engine as the graphical layer on top. First of all reach's engine has its own physics, gameplay, graphical, etc. Secondly they wouldn't need to bring in a different studio to build a custom engine (borrowing somethings from reach as you can obviously see) for the game.
Thirdly, its bringing 7 Maps to the Halo Reach Multiplayer (1 firefight, 6 multiplayer), along with a Title Update. Not all core CE maps.
If you have played Reach multiplayer recently, they have a "classic" mode that turns off armor abilities, alters the lifebar, shield charge and weapon damage to mimic H:CE and it does very well...they even did a bunch of remakes of H:CE and H2 maps in forge world for it and they are pretty good also.
It is very likely that this is a test-bed for throwback gametypes to be included with H:CE Anniversary.