Thinking about this--this isn't so much a skill system as an alternate way to use the systems D&D (and games like it) already have to adjudicate situations when you want to add tension.
Skills (or "Noncombat tasks that a PC might reasonably specialize in").
Simple skill is like: there's a task and a number you have to roll, roll over, roll under.
Sometimes, though, you want to make the execution of a skill as dramatic and tense as combat.
Quick review: Why is combat drawn out in most RPG rules?
1. The stakes are generally high (life or death)
2. A partial success or failure is meaningful (injured foe or friend--this injury matters even after the combat's over)
3. The task and subtasks it consists of are fun events in-themselves, even outside of any context. At least if the DM is doing it right. (You dodge, you swerve, you slash and lose a finger and all that.)
In contrast to fighting, D&D skills in the basic rules tend to be simple--pass/fail.
Type IV introduced skill challenges, of which I know little, but which, as a player, I have experienced and not really noticed having an effect much--though I assume the idea is to create just such high-tension skill moments. Crossing the tightrope, ooo, nope, whoa, ok.. etc. the general scuttlebutt on them seems to be: the quality of them is highly dependent on GM skill.
Anyway here's my idea:
Each test either has a difficulty class (DC) like in Type III D&D (this is just like an ascending AC. A number you have to roll w/modifiers to do the thing) or if you are using a roll-under-attribute system it has a bonus or penalty associated with it (make a dex check at -2 or +3 etc.). Both are ability score based--int for making stuff, dex for tricky physical tasks, str for swimming, etc.
Like a hard task could be DC20 or it could be -5 to attribute check.
(This is all old news. This is how people already do stuff in versions of D&D.)
If you don't have the skill, it's just pass/fail. You succeed, good, you have made a tea kettle or climbed a wall. You fail, the bad thing happens, the lock seizes up, the knot uncurls, the trap fails to spring, etc.
IF you DO have the skill and you succeed, well, good. You succeeded just like everybody else.
(Still, all this is old news. Here's the new bit...)
If you DO have the skill and you fail, the DM rolls d6. If the die is over your current level, you fail. If it's equal to or below, the DM narrates some mayyyybe-it's-working type situation ("the lock is rusty but a tumbler is beginning to come loose...""you slip suddenly, but catch hold of a badger-shaped rock at the last minute..") and you have two options:
1. You can try again.
2. If the task is the kind where you can start doing it and give up half-way without catastrophic disaster (i.e. you make it half-way across the chasm and then turn back rather than just falling) you have the option to "chicken out" and stop rolling now and have things go back to more-or-less how they were before the failed roll.
If you try again:
If you succeed, good.
If you fail, the DM rolls another d6, if this d6 plus the previous d6 is over your level, you fail. If it's still under, you can try again.
etc. etc. until you run out of levels or succeed.
This means by 6th level you will obviously automatically get one reroll. By 12th level you automatically get 2 rerolls. (No need to keep track of this, the dice will make it obvious.)
Why a d6?
I figure 6th level represents the point at which you obviously are definitely reliably always a little better at your specialty than the average PC. (This is going by thief PCs in AD&D--until about 6th level you still kind of suck much of the time and using your abilities is often a bad idea.)
In addition to the tension factor, I also think this system will scale better at high levels than simply giving skilled PCs one-re-roll per level or +1 to the roll per level--After 4thish level everything is way easy unless the basic task has such a high DC or negative modifier that nobody unskilled could ever try it. In other words, doing it that way means you have to have a lot of walls that only a skilled person could climb, and even then you keep having to scale up the difficulty as the PC gains in power.
This system means that (since the basic roll is still just vs. unmodified attribute) a skilled PC is measurably better at doing things than an unskilled one, but does not live in a world where s/he'll start automatically succeeding and need more and more difficult tasks just for the game to justify having the PC have the skill in the first place.
(I mean--if there's a wall in a 10th level dungeon only a thief with a skill can climb then that means either the party has to have a thief or the players know on the metagame level that there must be another way in. Plus I also just find degrees of awesomeness beyond, say +6 difficult to really appreciate on a visceral level. If you get to a point on a d20 system where some PCs are at +4 and some are at +9 and anybody below +5 is actually kind of crap, relatively, I find it hard to imagine what that's like in real-world analogues. What's a +9 guy look like compared to a +15 guy? +15 vs +20? It gets kind of abstract and numerical.)
Plus no skill points to keep track of in this system. Though I like the LOTFP 6-point system, it's not compatible with the have-it-or-don't skill system already in my campaign.
So, anyway, this seems like a decent bolt-on for any have-it-or-don't skill class-and-level system. It requires no new things on a PC's character sheet.
And again, I wouldn't use it for every test, just when I wanted to add suspense when the consequences are a big deal.