Only became shit when MS pushed for H2 to be brought out before it was completed.
Heres a story:
Microsoft was getting the push from fans and other annoying arseholes to get halo 2 out before release. Bungie, originally penning the story arc as a two game story (due to the huge success of the H:CE) were put under pressure from microsoft (their sponsors) and got the game out way to early, as can bee seen by the shitty button glitches everyone exploited. Bungie had to leave the last 3 missions out of H2, which would of finished the story arc, and because of this had to stretch this missions into a whole new campaign.
Once halo 3 was pumped out Bungie immediately separated from MS and started developing Reach in secret and ODST. H:Wars was developed by Ensemble Studios but tried to stay in the same Universe or story line as Bungie had conceived it. Wars was a flop due to being released onto a console, and Reach, by the sounds of it will bring the story arc back to the beginning where the Spartan II's were conceived with a new playing style (Probably squad based due to the subtle hints and the line of Spartans silhouetted against the planet Reach.
ODST will be a expansion of sorts to the H3 experience, bringing in a popular feature called FireFight. Similar to survival types, it adds extra spark by throwing in points and skulls to mess with peoples playing styles.
Bungie is also keeping an eye on Halo Legends, the new Anime series conceived by MS. If something goes amiss, i can assure you Bungie, and its members, will Boycott MS until H:L is fixed or removed from our existence.