Though Paulo's explanation was very thorough, as you said more opnions are welcome.. let me try to sum up what Paulo wrote: 

           Learning plateau is a period of time that, no matter how hard you study, you seem to get nowwhere, stuck, not gaining any ground at all.

           Based on my onw, very short, experience though....I’m of the opinion that it doesn't exist at all. I assume in real fact it stems from the student's head. They mistakenly hang on the idea that their not making any progress since they're not seeing solid results . Who haven't heard teachers saying these phrases: "The more we study the more we learn"...."Practice makes perfect" "? So, how can this "learning plateau" be true?

            I believe that it was Albert Einstein who once stated that insanity is doing the same thing over and over but expecting to get different results each time.

            Changin' our learning routine, coming up with new methods and approachs to improve our knowledge might help us to get rid of this fake feeling.........I think...:)