I read this so long ago that I had zero memory of it. Nothing, not the beginning at Oxford, not the middle where everyone desperately tries to return Kivrin to the current time, not the ending, which is so satisfying as well as unexpected in many ways.
What I found unexpected was the religious overtones of the book. I would bet that if I were to be sent back to one of the greatest tragedies in the world's history, and made to try and survive it, I would believe in things I didn't even consider believing in before I left. I am being deliberately vague so as not to try to give away one of the largest plot twists in the book (which I think is written a bit for show, so that's it's impossible not to guess at it). Even if you guess it beforehand it does nothing to lessen the impact of the final chapters.
I'll admit I do find Willis to be repetitive. The constant "oh, I must get Kivrin back" and "oh, I have to find the drop so I can return" becomes wearing at times. It's not that things don't happen, but especially in Kivrin's time, they happen darn slowly. So repeating things doesn't help the reader. It makes you wonder why the book is really so long. (My guess, she could have lost 150 pages easily.)
But hands down this is one of my favorite sci-fi stories, in its depth of research, in how you care for all the characters (even those that drive you crazy) and in how she weaves a heartfelt and thoughtful masterpiece out of one of our darkest hours.