Where do I begin, well the gameplay was certainly passable but there was a major problem, points of no return, twice. The first time wasn't really necessary and made it easy to miss Djinn, the second time was maybe a bit more necessary because you kinda screwed over the world with that eclipse.
The gameplay was too easy though, things don't even get relatively challenging until the post-game challenges like the return of Star Magician and Dullahan,
That's the minor problem though, the major problem was that too much of the plot was completely pointless, to the point that things get in the way of the actual plot which ALL gets pushed off onto the next game.
A synopsis: Dumbass friend breaks a flying contraption needed to investigate Mt Aleph so they can investigate a problem of Psynergy vortexes. In the quest to get the feather to fix the damn thing, the bad guys put you through a ton of shit that gets in the way, and leads into activating a dark eclipse which kinda destroys the world whilst it's active.
You finally undo the eclipse and everything's okay again, except we never learn a damn thing about the villain's motivations or why they coerce you into going everywhere they wanted, and whoops, we kinda forgot about those pesky Psynergy vortexes.
Not cool, when you look at it this way, this game can actually be completely skipped over as you never did anything you set out to do! You only had to undo the crap that the villains guided you into doing at all. In other words, Golden Sun 4 would actually be telling the story that should have been in Dark Dawn to begin with.
Now let's talk about your party members. Starts off alright, you have your Earth, Fire and Wind users. Later on at the first point of no return you get your first Water user, he's kinda useless though since he's a healer in a game that really doesn't need healing psynergy, just some healing items. But very soon after that you get your second Water user anyway, who actually works on an offensive front. Later on you get your next Wind user who is basically princess of furry town. She's important and likeable, she's well motivated and great in battle too, probably broken even. The next two party members felt like filler though to fill the role of 2nd Fire and Earth users. Your 2nd fire character joins up because his father dies as a result of the Dark Eclipse bullcrap, and your 2nd Earth character is a walking, barely talking plot device. Literally, she is awakened by a key item you grabbed and joins your party so she can use a single, plot-advancing psynergy. She never takes part in group discussion or interaction, isn't that exceptional in battle and isn't even well motivated, she just goes with it because it's her destiny after waking up from a very deep sleep.
I'm not as well-versed in Golden Sun lore so I do forget much of the significant locations from the past game, many seem to disappear from this game though as the geography of the world has drastically altered because of the Golden Sun event, to the point that some animals because half-human and some humans became half-animal in one town. I don't really have as much of a problem with this but I've heard of a few people being disappointed on this front too.
One last thing, the game can't stop reminding us that Isaac and his team were the great heroes of the world, re-enact the final boss of The Lost Ages, collect all these story-telling books, etc. Gets kinda annoying, more so when only Isaac and Garet even make appearances, the rest are barely even mentioned and seem to hold no stake in the story.