[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?
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.