Sting was turned heel because whoever was booking at the time wanted to swerve us. I thought it made sense from a storyline perspective, but the problem was nobody was going to cheer Hogan over him.
Luger turned heel during the fingerpoke incident and was injured soon after. He came back a few months later to play Sting against Hogan, which he ended up doing. Then he and Sting ended feuding at the end of the year.
Hogan and Flair had a "double-turn" at Uncensored, though Hogan was more of a tweener until he got injured. Then he came back, and ended up allied with Sting and Goldberg against Nash (who turned heel for some reason, Sid, and Rick Steiner).
Bret was injured for most of the first few months of the year, but was a heel the whole time. He turned face after Owen's death, returned in the fall and pretty much wrote the wave of sympathy until his nonsensical but totally predictable heel turn when the NWO reformed.
Sid debuted as a heel to help Savage. Then he was allied with Rick Steiner and the Outsiders until Sid turned face and feuded with Nash. He remained a face until his aborted feud with Hogan in the spring of '00.
DDP turned heel because Nash wanted to win the belt as a face.
Yes this was the year Flair went to the loony bin. He turned heel at Uncensored and I think he went face again sometime in the fall before disappearing for awhile.
Goldberg was around in the upper midcard.
WCW went ass-backwards because it's WCW.
Hall was out most of the year. Nash went from face Wolfpac leader to heel NWO elite to top face to heel in order to fight Hogan to retired to Hall's manager to active wrestler.
Benoit and Malenko were Horsemen, turning heel when Flair did. They turned face again to set up a young vs. old storyline that primarily pushed Buff Bagwell. Then they formed the Revolution with Shane Douglas and Saturn. The group soon turned heel as Benoit went solo and feuded with the Filthy Animals which included Eddie Guerrero who was out much of the year due to a car crash.
So in summary: injuries, Bischoff leaving WCW in the hands of Nash, and Russo having too much freedom added up to a year of crap.