The lack of communication and trust between nearly all the different characters drove me nuts. Especially when I'd guessed most of it from the beginning and I couldn't believe it took over 15 years to sort out. I also feel that Nick should have come back way before then, if he really meant everything he said! At least he should have gotten in contact with Alix. 
 However, The Sweet Gum Tree was an intensely emotional read and as a result it was very gripping.