[Inspired by the What is meant by "steep learning curve"? question]
Is there a phrase similar to steep learning curve (i.e. [some-word] learning curve) describing something that is easy to learn?
How about gentle learning curve?
A "gradual learning curve".
You can Google the phrase - it's used all over to imply the opposite of "steep learning curve".
I prefer flat learning curve.
It does not have confusing positive connotations (as in 'gentle' or 'smooth') as mild learning curve is a Bad Thing. Flat means horizontal, ultimately never reaching the value L(x)=1, implying, this is something not possible to learn, no matter how hard you try.
Just for completeness: As I answered on the 'steep learning curve' question, "steep learning curve" itself technically means something that's easy to learn.
(Admittedly, unless you're going to explain this, or are writing for an appropriate technical audience that understands the phrase in this sense, it is advisable to use a different phrase.)
I'd agree that 'Gentle' is a suitable antonym for 'Steep', but perhaps one might tackle the problem in a different way.
Perhaps, rather than having an 'antonym-of-steep learning curve', it is stated that the subject 'doesn't have a steep learning curve'? My point is, if figuring out the most appropriate opposite was a matter of some debate, perhaps it's easier to negate the original phrase... a phrase that is universally recognised and understood.
To my ear mild learning curve sounds best.
I often hear "smooth learning curve"
If you think about the verticality of something you could have a "level learning curve".
how about minimal learning curve
soft learning curve
no learning curve
light learning curve
benign learning curve
genial learning curve
moderate learning curve
From the above posts (which I must admit are quite well technically reasoned out), I am inclined to go for the near literal opposite of "even curve", if anything like that can be found in the vocabulary of the field of study — just for clarity purposes.
Personally, I say "protracted learning-curve" or "shallow learning-curve" to describe the opposite of "shallow-learning curve." Thank ya.