I won't list the differences between this and Broadchurch, as others have already done so on the Gracepoint IMDb page, but these are all, by my estimation, quite minor. The new or enhanced backpacker and the priest story lines were of some little interest, but ultimately didn't contribute much. (Particularly when you realize that the 2 additional episodes you get in the USA version vs. the UK version were meant to have existed before, and the Brits cut them for cost; meaning, these aren't a new addition, just a restoration.) Even the big ending 'whodunit' twist that was advertised, to my mind just came too late to do much other than fall flat, in terms of storytelling.
I liked the actors that played the priest and Beth well enough, and the daughter and her boyfriend were perfectly tolerable, but otherwise I found the acting to be painfully wooden---particularly Ellie Miller's character... and even my much beloved David Tennant. Maybe he was just too focused on the American accent to act as well as he is able, but side by side, the Broadchurch performance was nuanced and powerful, and this one felt more like a rote recitation of lines. I just don't understand all the positive reviews of people who've seen both.
I almost NEVER have totally negative things to say about TV shows or movies; I can always find something worth seeing, but this? Flat, stiff, bland. The soundtrack was pleasing. Er, the whales were nice. My recommendation is to give this a pass entirely, and watch Broadchurch instead. And remember, that's two series' long, so you get the crime solving and the trial, which was amazing in its own right.