Almost entirely rebalanced prequel that plays very differently from any other game in the series.
The enemies are much stronger and faster than in previous games which causes combat to be very frustrating. The elites move CONSTANTLY and at such a fast pace that casual gamers will have trouble tracking them. Beware melee combat with the elites as well, because they will kill you much faster than you kill them.
Weaponry is similar to previous games so previous experience counts for something, but the guns feel underpowered due to the toughness of the enemies. About the only weapon to become more powerful are the grenades. The explosion radius has been increased and can kill at a much greater range.. Pay attention to where they land because they will kill you much easier as well.
One cool feature that was added was the pick-up abilites. Most of them are quite limited in their utility, but they can be fun or useful at the right time.
The campaign mode was too short (or perhaps too long for it's difficulty). Overall the game would be good if it weren't part of a great series, but because many gamers will be playing this after becoming accustomed to different gameplay, the changes can be unwelcome. If Halo: Reach is played with a completely open mind it can be enjoyable, but not so much as its predecessors.