Doesn't seem much until you play it.
This game is so underrated that I think reviewers played it in secret so that they won't get caught and lose their jobs :P ... well let's begin the review.
Starting with the story:-
Well it's one of those games that feature Luigi and some Mario so what do u expect the story to be? it's as simple as Luigi winning a mansion from a contest he didn't enter and goes to see it only to find Mario kidnapped and the mansion full of ghosts! right at the beginning it is clear that the game was made as a horror game for kids with cartoonish ghosts and a scary heroe and it appeals to hard core guys by providing a nice mansion to explore and laugh at it's ghosts with some startling moments. Besides Luigi who you start with you meet with Dr.Elvin Gadd who lives in a garden hut outside the mansion and you meet with tons of toads to save your game and light up the rooms you meet them in.
Other than those friendly characters you meet with lots of bosses each with a unique personality giving you a chance to get to know the mansion's Ex.Members, you even meet with their dog!
So you start playing when Dr.Gadd provides you with the Poltergust 3000 your trusty vacuum cleaner integrated ti suck ghosts and store them for Dr.Gadd to turn them into a paintings and place them in his gallery ... not so human I say :P
So how do you top such a story? with a good presentation ofc :P
This game's presentation is very good keeping in mind that it's a launch title. The textures has good quality and there is a good level of detail for each room and hallway. The only drawback I can see is the fact that most objects will react in the same way to vacuuming or knocking or tries to move them.
In terms of sound the game has good sound effects for all gestures and the soundtracks in this game are unique since they all follow the same tone but are played in different ways which keeps them fresh in the mind and not boring since you'll hear the main theme remixed a ton of times for each situation or place. Of course there are other themes for bosses and other places but the main one is the most memorable .. it's just something else.
More toppings come with the gameplay.
The gameplay here is also unique to this game. You don't kill ghosts instead you capture them by pointing a flash light at them to expose their hearts and then take them in to your vacuum and to that you need to keep sucking and pulling in the direction opposite to their movement until their HP runs off and then you got 'em. There is a good variety of ghosts and ways to kill them. Some ghosts require you to shoot water, ice or fire at them and those abilities become available at certain points through the game and can be replenished by sucking elemental ghosts from places where they live like water fountains and candles.
Bosses on the other hand are creative and need a different strategy each varying from tossing balls to answering questions about super Mario bros 3's theme song but usually they are not hard except for the last boss .... he's a tough nut to crack.
So to sum all that game up: it is a cool game with some innovative mechanics and while most people say it's bad I think they did a wrong thing cuz this game is COOL!