What is the difference of Chameleon 100% between Morrowind and Oblivion?

It appears to work similarly, though at a lower percentage.

Oblivion's Chameleon at 100% (emphasis mine):

It is possible to combine multiple Chameleon enchantments to reach 100% Chameleon, causing your character to be visually non-existent and nearly completely undetectable while performing any actions. NPCs will only interact with you if they are scripted to by a current quest or if a guard is attempting to arrest you. Values higher than 100% are possible, but do not provide any additional benefit. With 100% Chameleon, you are able to move freely throughout the game, killing, stealing, sneaking, and doing almost whatever you want without ever being seen or confronted. You are basically invisible to all living creatures or people within the game. However, you can still be damaged by items or traps.

So essentially in Oblivion, 100% Chameleon, as you stated in the question, makes you basically invisible, allowing you to get away with whatever you want.

A similar effect appears in Morrowind (emphasis mine):

If the Magnitude of the Chameleon effect is high enough, no object will be able to spot you under various conditions, seemingly based on Fight rating. 75% Chameleon seems to be sufficient to walk right past hostile NPCs without issue.

So it seems that the same ability to move around undetected exists in Morrowind, though unlike in Oblivion, it occurs at 75% Chameleon (as opposed to 100%).

Chameleon plays a number of different roles in Morrowind gameplay:

1. If your Chameleon rating is 75 or greater, NPCs will mostly not engage in "casual interactions" with you: they won't turn to face you as you walk by, they won't initiate dialog, they won't attack you even if their aggression level is high.
2. It factors into your ability to sneak, but there's no hard threshold above which Chameleon makes it impossible to fail. Rather, you gain sneak bonuses from your luck, agility, sneak skill, Chameleon level, and distance from an observer. These are countered by the observer's luck, agility, sneak skill, blindness level, and the weight of your boots.
3. It factors into your ability to dodge hits in combat. Mods can change the strength of the effect, but by default, you need 500% Chameleon to get the maximum effect. Even with the maximum Chameleon bonus, it's possible for an attacker to accumulate enough attack bonuses to completely negate it.

Chameleon in Morrowind is a powerful effect, but unlike in Oblivion, there is no threshold above which it acts as an "I win" button.