There are lots of secret characters in the game, actually for every standard character there are 3 secret ones that must be unlocked in succession (beat the game first with the regular, unlock first secret character, beat the game with the first secret character, get the second...). So that makes 4 tiers of characters available. Tier 2 has wrestling celebrities and legends, tier 3 goofy-fantasy characters like santa claus, and tier 4 has...the developers!
WCW Nitro for the PC is an original one. Actually Playstation's Thunder is a tuned version of PC's Nitro: Thunder has weapons, cage, more real-life finishers. Also Thunder was released AFTER Nitro (PC)
On the character select screen you can press a button to make each of the characters tell you why you shouldpick them over the other one.
This game has the same characters as the Playstation version of WCW Thunder so is basically that and not WCW Nitro.