100%'d last night. Maybe some very minor spoilers ahead, but nothing that would 'ruin' the experience for anyone.
Anyway, S8-Crown took me about 150 attempts to beat w/ both characters. Probably the most challenging thing I've ever done in any Mario game. It was way more challenging than the Grandmaster Galaxy Daredevil run in SMG2, yet interestingly enough, it didn't feel as frustrating. This is the only stage where I noticed the somewhat random feeling of the acceleration/physics due to how precise some of the areas were in terms of platforming demands. I could literally feel my pulse pounding as I neared the end. Unbeatable feeling of accomplishment, too. Amazing.
I would say that compared to the Galaxy games, the overall feel and tone of SM3DL is somewhere between 64 and Galaxy, but there are nods to a lot of past and present games in the series. The soundtrack is definitely a lot less epic than SMG (so spoiled by those OSTs), and isn't nearly as memorable, but it fits the whimsical nature of the game well.
The one thing I wish they would've done is give you a new set of rearranged Star Medals for the Luigi playthrough. As it stands, the Luigi missions are little more than speed runs in order to unlock S8-Crown above. That would've easily doubled the length of the game, but you can fly through all 16 worlds with Luigi pretty quickly (I did it in about a day). But after playing S8-Crown, maybe this was the right design choice.
Anyway, pretty minor complaint... overall, this is the best portable Mario game I've ever played. It's really polished, fun, and definitely legitimizes ownership of a 3DS for me (in addition to all the other recent reasons).
Game of the Year for me.