What makes the game so great is that Bungie increased the power of their forge engine. Now with the "snap" feature you can click blocks seamlessly together. Second you can now merge blocks together without a difficult merging process like the one required for Halo 3. The game is far more friendly to forge with and even people who dislike the forge of Halo 3 seem to enjoy the new features of Halo Reach's forge.
My personal favorite part of the game right now is the fact that your multi-player character is no longer just someone you see only in free-for-all matches. Instead he appears everywhere! You play as your character during the campaign and during firefight (more on that later) being able to actually have your player count toward something makes a big difference.
Firefight a feature available since Halo 3: ODST has been a fun addition to Reach. It allows players to fight against endless waves of foes pounding against the enemy and striving for victory. The addition of Firefight is a fun one indeed allowing players to hone their skills against a constant rush of enemies. My only complaint on the Firefight mode is that you can not play it on forged maps, this would have been awesome and allowed for so much possibilities.
The community of forgers that developed hundreds of maps for Halo 3 is now creating for Reach! Forge Hub is a forum of some of the most skilled creators that don't belong to Bungie and they have created hundreds of amazing maps for both Halo 3 and Halo Reach. If you are interested in race maps I highly recommend Jelly. This race map is one of the most difficult and fun to drive courses I have played and requires a lot of skill to master and even complete.
Halo Reach