Mustrum_Ridcully
Legend
You could see it as something that sets a feel for the power of characters in 3E. 8 ranks is more than just modest training...Disguise lets you impersonate a specific individual, and the skill description lists the modifiers other NPCs get to their sense motive check, based on how well they know that person.
If they know the person and would recognize him on the street, it's +4.
If they are personally acquainted, it's +6.
If they are friends, it's +8
If they are close family, it's +10,
For a regular person, that's a Sense Motive roll of 11 to 30 to recognize someone impersonating your father or wife.
A 5th level bard with 16 Charisma could have +13 to that Disguise roll without even trying any optimization shenanigans. That's a more than 50% chance to pull off something that should be impossible with only a modest training. Cast a simple 1st level disguise self and you boost your roll to +23. Even if the person rolls a 20 on sense motive, you still only need an 8 to pull it off.
If a Level 5 Bard can disguise himself so that they can convince a husband the bard is his wife, than that tells you how powerful Level 5 really is compared to the real world. And maybe it makes sense it would be that exceptional, because a 3E Level 5 Fighter can fight a pair of Ogres or a single Troll, which seems also extremely unlikely to be doable for a single warrior.
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What I remember from the skill list is that all these skills had the same "value" in character cost, but some were very specific (Read Lips) and something you'd barely used. And other times abilities that you would expect to go hand in hand needed and could be levelled completely independently. You could theoretically have someone that sneaks around and is almost impossible to see but you'd hear him a mile away (or vice versa).
Of course, the granularity of skill systems is always a challenge and this was basically D&Ds first fully-fledged skill system for all classes.