My strategy is to ask "Who should be expected to succeed at this task."
I've decided to go to a multiplier modifier rather than an addition modifier for easy/difficult tasks, although addition modifiers apply afterward. So an Easy task would be at double chances, a Very Easy task at three time chances, Difficult at half chance, Very Difficult at one third chance, etc.
(I think I'll ask the players to each bring a reliable calculator to the session.)
So a normal skill check is one that a normal person could succeed or fail in, like swinging a sword (or perhaps climbing a tree right now to get away from the bear in a single combat action). A skill check that a professional could almost always succeed at, such as using a piece of professional equipment, would be Easy (double chance), assuming that most professionals have at least 50% skill in skills needed for their profession. A skill that most people should succeed at if they've had training (such as driving across the country without an incident) would be Very Easy, at triple chances.
I should probably define terms for five times, ten times, one fifth, and one tenth chances.
This also provides a guideline for how many skill points a character should have. NPCs should have at least 50% in skills needed for their profession. In Modern Runequest, I've considered that High School graduates in the U.S. (most of whom can be assumed to have passed a Driver's Education course) have +10% skill in Drive Automobile, which added to the default of 10 + POW means the average High School Graduate can be expected to drive across the country without an incident.
The last case shows that there should be some GM interpretation here. Given the probabilities, perhaps it's reasonable to have PCs with less than 33% in Driving make a Very Easy roll to avoid an incident. If an incident occurs, then a normal roll would be indicated to avoid an accident.
Another rule I've adopted is that for an Easy or Very Easy task, if your effective chance is above 100% you don't have to roll. Normal and Difficult tasks have a minimum chance of failure.
(One option might be to have Normal tasks have 1% chance of failure, Difficult tasks have 2% chance of failure, more difficult tasks have 3%, 5%, or 10% chance of failure, based on the divisor used.)
To answer the original question:
A "normal" task is one that normal professionals have a good chance of failing.
An "easy" task is one that normal professionals have almost no chance of failing, but laymen would have a good chance of failing. (+50% modifier, if you prefer addition to multiplication)
A "very easy" task is one that most people with training could be expected to accomplish without failure. (+70% modifier, if you prefer addition to multiplication)
An "extremely easy" task is one that almost anybody with the most rudimentary training would be expected to succeed (five times chance, or +80% modifier if you prefer addition to multiplication)
A "difficult" task is one that would have a significant chance of failure, even the leading experts in the world (half chance, or -50% if you prefer subtraction to division)
A "very difficult" task is one that even the best experts in the world would have a significant chance of failure (one third chance, or -70%)
An "extremely difficult" task would be one where the best in the world have a significance chance of failure (one fifth chance, or -80%)
A "critically difficult" task would be one that requires a critical success to succeed, otherwise it fails.
PCs and NPCs should have at least 50% in skills that are essential to their profession, and at least 30% in skills that they consider important to their characters (talented amateurs, etc).
I hope this helps.