Well back in the “old days” of the 80s and 90s, it was apparently quite hard and only a very small percentage of people would be asked back. The fact that Darren Aronofsky couldn't make the cut should give you an indication of how high the standards were*!
But things have changed since then and the process isn't anywhere near as stressful. It depends on the discipline as to how thorough the review is. As a screenwriter, we didn't even have one and I don't think any writer has ever not been asked back at least since the current faculty took over. One director wasn't asked back, as well as one producer, two cinematographers, and I believe two production designers. There are other fellows who elect not to come back for various reasons (usually financial), but people being “cut” is less about pure talent and more often comes from their having a poor attitude and being unwilling/unable to work with their peers. If you attend class, treat your classmates well, and work hard (even though you will make mistakes), then you have a very good chance of being asked back for the second year.
*Though in all fairness, if you watch the film he made his first year you can see why the faculty viewed him as someone not worth keeping.