I just finished up the first mission, time for first impressions.
Straight away, this is an amazing looking portable game. I haven't experienced a visually wowing portable game since God of War Chains of Olympus on PSP. The color palette is broad, vibrant...all of the ghostly neon pops wonderfully on screen. I'd argue that Dark Moon looks nicer than the original GC game. I'd also argue that it's the best looking game in the 3DS library.
On controls. Since the original game was a twin stick game I was worried that the 3DS's single stick would hamper gameplay. Although it's early I've had little trouble using the face buttons to look up and down. Give it a few minutes and it'll feel natural.
On audio. Usually one of the main reasons behind my liking or not liking a game, Dark Moon's audio does not disappoint. It perfectly sets the game's mood as a horror/adventure. You'll even occasionally hear Luigi happily hum along to the background track, as though he's trying to calm himself. Thunder rumbles, ghosts cackle, and backgrounds environments offer pseudo-realistic sound effects.
Needless to say, but Dark Moon's curb appeal is fantastic.
I'll edit with more later on as I play through the day.
Edit: just finished up mission A-3.
This feels like a horror funhouse. Haunted house tropes have been woven into how the player progresses through the mansion. 19th century small service elevators, movable spaces between the walls, etc. I get the sense that I'm in a haunted house you'd find at your state fair or an annual Halloween farmland conversion scarehouse.
Sadly, not all is great. The 3D adds nothing to the atmosphere set by the audio and visuals. I'd even advise against playing with 3D on as it causes the graphics engine to produce some seriously ugly jaggies. Both in the background and on character models.
Each mansion is broken up into missions, all are replayable. Which provides backtracking, allowing you to use abilities unlocked later in the game to obtain collectibles you previously couldn't. Replayable missions also allow you to grind weapon upgrade levels, making your Poltergeist 5000 increasingly more powerful. There're also golden dog bones and purple gems to collect but I don't know what they're all about just yet.