While some people decry using skills in D&D and D&D-clones as 'modernization', using ability checks and the x-in-d6 chance for things has existed since Original D&D first came out way back in the 70's. There were no Thieves in that very first game (they being added in Supplement I: Greyhawk) and all characters could perform 'dungeoneering skills' like sneaking and picking locks.
My first skill system for the World of Onn used a d12 with a 1-4 chance of success instead of a d6 with a 1-2 chance of success, because I like using modifiers for conditions, difficulty and ability scores (as a side note, this can make things as simple or as complicated as you let it be). Through the playtesting though, I found it easier to have a specific target number (much like the modern 'DC' D&D 3.x and 4 use). In addition, it just makes things easier to figure out on the fly when dealing with running a modern module and using Onn (or Swords & Wizardry).
I still use a Target12 system for my skills, finding a d6 too small to use with almost any modifiers, but a d20 way too huge and leading to skill bloat. Besides, the d12 is like a red-headed stepchild and needs some love (or maybe it's my odd liking of things...odd).
Relevant Ability Score
Of course, keeping Onn a retro-style game means the skill list should be small and these are the ones that have been the most relevant in both campaigns I've run. The skill challenge targets I codified in the following manner:
Dungeoneering Skill Targets
5 or less
*Only roll if character is under duress or failure has major impact, otherwise use your judgement.
This was done for two reasons: 1) since all the new material is written for a skill system like 3.x/4e it's easier to have some form of conversion written down to keep things consistent; 2) Having a solid targtet number to work with keeps the game flowing smoother if you're using any skill system. The Target12 system still uses a d12 to roll the check and is basically the following formula:
d12 + any racial adjustment + relevant ability score adjustment + misc adjustments
If you equal or exceed the target number, you have a success. A natural 12 is also considered an automatic success and a natural 1 is an automatic failure. As noted many times though, the Referee should use his judgement where skills are concerned. If a character has set himself up to succeed by creating favorable conditions and/or is using the proper tools for the job (with a proper amount of stress-free time), then generally they should succeed at any difficulty of task eventually.
One may also notice that there are no social interaction skills. Charisma can be used to affect the reactions of NPCs and monsters and the degree of success (or failure), but a die roll should never be used to replace the social aspect of a ROLE-playing game, as discussed many times at length (such as the last post and across the OSR blogosphere).
I may use this expanded version of skill challenges in Onn's 2nd printing, but even though it works well for me I don't know if the feel is right for the retro-feel I want Onn to have.