RPG System - Core Resolution Mechanic

So I dug up some notes I created a while back on an RPG resolution system that just uses D6. No doubt it's similar to some existing games already out there, but for posterity I thought I'd note it here.

Basic Roll Mechanic
Roll D6's equal to the skill or attribute (typical range of 1 - 5). 
If this value is zero, roll 2 dice and discard the higher one.
If less than zero, also reduce the rolled die value by this amount. 
Any die that exceeds a Roll Difficulty (RD) value is considered a success.

RD 1 = Trivial
RD 2 = Easy
RD 3 = Demanding
RD 4 = Difficult
RD 5 = Very Difficult (requires at least 1 six)
RD 6 = Extremely Difficult (requires 2 sixes)
RD 7 = Insanely Difficult (requires 3 sixes)
RD 8 = Improbable (requires 4 sixes)

Each 6 set aside from the rolled results reduces the RD by 1. This is the only way to succeed with RD's of 6 or higher.

If no results meet the RD, examine the highest rolled value (including any 6's set aside). 
If it is equal to the RD, this is a failure. 
If it is below the RD, this is a fumble. The amount below the RD is the degree of fumble if it is needed.

You cannot get more successes than your basic Skill or Attribute value (regardless of the rolls). You only fumble if no failures or successes were rolled.

Opposed Rolls
Each participant typically rolls against an RD of 3 (though this can be higher or lower depending on the circumstances of the contest) and compares the number of successes. The difference is the actual degree of success for the competitor with the highest number of successes. Note that fumbles count as negative successes (so a roll of 2 fumbles vs a roll of 2 successes equals a total degree of success of 4 for the victor).

Task Rolls
Tasks take Time
Tasks have Progress

The Time aspect of a task is how often the character gets to make a roll.
The Progress aspect is how close the character is to completing the task. The success of the roll determines the amount of progress gained.

The number of successes squared is added to Progress.
The degree of fumble squared is deducted from Progress.
Negative Progress indicates the current task can never be completed (something breaks, goes disastrously wrong etc.)

A standard Progress requirement would be 10 points of progress.

Time Steps

• 1 Second
• 6 Seconds
• 1 minute
• 6 minutes
• 1 hour
• 4 hours
• 24 hours
• 1 week
• 4 weeks (moon)
• 13 weeks (season)
• 1 year
• 10 years
• 100 years
• 1000 years
• 10000 years

A character may reduce the Time Step by 1 by increasing the RD by 1.
A character may reduce the task RD by 1 by increasing the Time Step by 1.

Example Task - Lockpicking
Time Step: 6 seconds
Progress: 10
RD: Complexity of Lock

Co-operative Rolls - Quantitive 
For quantitative tasks (such as lifting a heavy object) where the addition of extra help increases the effort that can be applied to the task, then simply combine all the dice of all the participants. However, after all modifications have been applied to the RD (ideally reducing it based on 6's rolled), check for fumbles on an individual basis - for example if helping to lift a heavy rock a fumbling character may injure themselves, even if the combined result was successful.

Co-operative Rolls - Qualitative 
For qualitative tasks (such as cooking some broth, or fighting an opponent) then each participant contributes as in quantitative tasks, but with 1 less die than usual*. In addition, any individual fumbles are removed from the total successes to determine the final result of the roll.

*If a participant only has 1 die to start with, instead of removing a die, they roll 2 and discard the highest, just like making a basic roll with a skill of zero.