Design Goal: Create a mechanic for character skills that allows for any class to attempt any skill while still giving Thieves the best chance to succeed.
Skills are a common sticking point when it comes to D&D. Most DMs have tried to tackle the problem of what happens when a fighter wants to move silently or a magic user wants to find traps. The goal here will be to give everyone a baseline chance to attempt a skill, generate some modifiers so that a character with exceptional abilities has that fact reflected in their skills, and still have Thieves be the best at what their class does. We will specifically focus on Thief skills here, but can expand this to other skills as well.
Every skill starts with a base 1 in 6 chance for success. Ability modifiers can be applied at a maximum of +/-1. Racial modifiers apply as detailed in each race's description. Let's develop each skill from there.
Pick Locks. Modified by Dexterity. Any character can attempt if he has lock picks.
Find And Remove Traps*. Modified by Wisdom. This skill is also a catch-all for detection skills. Elves would have a bonus here only if searching for hidden doors, Dwarves only if searching stonework, etc.
Move Silently**: Modified by Dexterity. Penalties due to armor worn apply: light armor -1, medium armor -3, heavy armor -5.
Climb Walls**: Modified by Strength. Penalties due to armor worn apply: light armor -1, medium armor -3, heavy armor -5.
Hide In Shadows: Modified by Dexterity.
Hear Noise: Modified by Wisdom.
For all of the above skills, Thieves receive a +1 bonus every third level. Thieves also have no penalty to their skills when wearing light armor.
We've achieved our Design Goal here, every character now has a chance to perform any skill, there is room for exceptional abilities to play a small role, and Thieves will always be better at these skills in the long run. The choice to add +1 every third thief level is to mimic and linearize the actual wacky %s listed in the Advanced Edition Companion. We sacrifice some granularity to gain some simplicity.
In addition, we roll a mechanic for armor modifiers into skill use. We also open up thieves with an added option to forgo their skills to wear armor, giving them a slightly better chance to survive combat. Of course, that choice still entails throwing away any decent chance at a move silently for a backstab in combat. A very high level thief would be able to even wear medium armor and still have a decent shot at their skills.
* These skills primarily work through character action. For example, if a character uses a pole or staff to prod the ground in front of him, he WILL detect a hole in the ground directly in front of him that is covered by a thin blanket of leaves and twigs. Using a roll is a catch all method for "I look for traps." Similarly, if an object is hidden under a bed, for example, anyone who says "I look under the bed" finds it. A roll for finding hidden items/doors is the catch all for "I search the whole room."
** These skills are modified by common sense. A guard who is napping is easier to sneak past and a rough wall with many hand holds is easier to climb.