I just introduced this book to our 3 year old today, and we've read it through 4 times already. There's enough action to keep him interested, and the illustrations are absolutely lovely. I don't know how Potter manages it, but somehow Peter is incredibly expressive yet still looks like a rabbit the entire time. I'm used to more current illustrations that are often so anthropomorphized that you lose the original animal somehow. Peter never looks like anything but a rabbit, but you still can see the range of emotions he experiences. 
 I also really enjoyed the the expressions in this. "Lippity lippity," if you say it out loud, sounds exactly like what you would expect it to sound like for a rabbit to slowly hop away. And how many children's books use words like "whom" and sentences like "implored him to exert himself?" None of that threw my son off at all (nor did the fact that Peter's father got put into a pie), so we'll be trying the other Potter titles.