My gut response is to say there are a limitless supply of these plateaus. There is so much out there that even the best mathematicians are limited in what they can understand well. So in terms of specific concept plateaus, well if you're like most of us you'll probably have lots of them, and that's a good thing.
In terms of concepts, I think what I found tough was often much clearer after I lost an early misconception. E.g. for a long time I thought the Killing form in Lie algebra was using "killing" as a synonym for "erasing"... I tried to build my understanding around that conception and it didn't work very well (Killing is a name). A lot of "simple" mathematical ideas are known by proper names rather than descriptive terms, so as more of these accumulate you have to rely more on memorization than intuition.
Outside of concepts, here's what I found tough:
- Transition from coursework to research. Some people are very good at getting the A when the material is put in front of them, and most textbooks are good at giving you the necessary tools to solve the problems they present. I found the transition to more open-ended problems a significant challenge.
- Understanding the frontier of a field. As stated in a previous response, it's tough to get to the frontier of a field. It takes a lot of work, and a lot of time. So graduate school requires a lot of perseverance.