Halo Reach offers much of the same in a new way and it's not bad at all.
Campaign:
The campaign in reach is very well done. It begins with minor skirmishes and slowly turns into an all out war with the covenant. I think it's the best campaign of the halo series. ( probably because there no flood :P). The campaign has some good moment and you soon realize that it is an hopeless fight for survival. The missions are memorable and Noble team has some memorable characters too.
Game play:
The core game play of reach is very familiar with halo 3. There are a few changes but generally it is the same, except that your health isn't recharging and that you can do assassinations. That is probably a good thing because the game play wasn't bad in halo 3 and the problems that where there has mostly been addressed. A new thing is the credit system. You can get credits by playing multiplayer and complete daily and weekly challenges.
Firefight:
Firefight is a nice break from the multiplayer arena, Here you can fight with 4 players against waves of covenant forces. You can change all kinds of options to make your own experience ( a good way to get achievements easily)
Multiplayer:
Multiplayer is almost the same ass halo 3. The only difference are the maps and weapons. But mostly it's the same. Here you can also earn credits for new armour parts. There are all kinds of modes from halo 3 and some new ones. There not a lot of maps but I think bungie will make up with that through new dlc.
Forge:
The forge is renewed in Halo reach. You now have a big map where you can get your imagination loose. You can place all kinds of objects to make you own castle. Another way to entertain yourselve with forge is to gather 3 friends and let them loose in forge and you can fill an entire night.
Conclusion.
Halo Reach offers much of the same in a new way and that's not bad at all. If you like a good campaign and solid multiplayer Halo Reach is what you're looking for.