Luigi is able to body-slam ghosts with the Poltergust G-00. And he can use the ghost he's currently sucking up to deal damage to other ghosts, too, giving him an edge in the fights.
Luigi and Mario briefly teaming up to climb to the roof and save Peach, just like in the old days. They even have the same dynamic in the Mario and Luigi series. Unfortunately, this is short-lived as it was ultimately a trap by King Boo to get everyone in the same place so he can trap everyone in a portrait.
The stare-down that Luigi and King Boo have as Luigi vacuums King Boo into the Poltergust. Luigi looks afraid at first, and even looks away for a second, only to glare back at King Boo with a renewed look of determination as King Boo is finally sucked into the Poltergust.
You gotta give points to King Boo in this game as it's clear he learned some lessons from his last defeat. At the start of the game, he captures all of Luigi's friends (including E. Gadd and the Toads) and ambushes Luigi while he doesn't have his Poltergust, only failing thanks to Luigi jumping into a nearby laundry chute. But even when Luigi meets with E. Gadd and makes this way to the hotel, he's still ready for him. He knew that Hellen would ultimately fail him, so he devises a clever plan to lure everyone into one final trap. He takes Peach's painting and places it on the roof, leaving Mario with Hellen. He knew that Mario, being Mario, would daringly rush in to save Peach on the rooftop after being saved himself and that Luigi would follow after. Meanwhile while the Mario Bros. are scaling the final portion of the hotel, King Boo makes his way into the basement and uproots E. Gadd's lab with E. Gadd and the Toads still inside. Finally, when Peach is reunited with Mario and Luigi, King Boo ambushes the three and reveals that he has re-captured E. Gadd and the Toads. He stuffs them in a portrait, and proceeds to drop it on the rest of the gang, sans Luigi thanks to a Diving Save from Polterpup, and if it weren't for Polterpup, King Boo would've won. This easily cements King Boo as one of the most dangerously competent villains in the Mario universe, who only lost due to dumb luck.
Right after getting rescued by Luigi, Mario is encouraged to follow him to the makeshift lab E Gadd set up to stay safe with the other toads. Does he follow him? Heck no! He encourages Luigi to follow him and rushes straight to the top of the tower, jumping over gaps and even wall jumping to get to the top of the tower! Only 10 seconds after getting rescued and he is up and running!
Polterkitty is considered to be such an annoying boss, that after spending minutes perhaps even hours going floor to floor trying to retrieve the elevator button she swallows, that it can be quite satisfying to see her get sucked up once and for all the second time around. Not going through the ordeal again for the rest of the game.
This is potentially even more satisfying for those who have played Luigi's Mansion: Dark Moon. In that game, Polterpup (who served a similar role to Polterkitty) gets fully sucked into the Poltergust numerous times, but repeatedly breaks out of the ghost gallery until it's repaired and made secure. Here, when defeating Polterkitty the first time, she isn't sucked in, but rather coughs up the elevator button and flees. Thus, when you see her actually vacuumed up the second time around, you know that she's gone for good.
Super Suction. Once you finally get this power-up and find a wall socket to hook it up, be amazed at the awe-inspiring power now under your control as it destroys everything in the surrounding room. Even the background music conveys a sense of awe and horror more befitting of a Card-Carrying Villain than a Lovable Coward hero, especially when used in one optional area. Unfortunately, it's such a Story-Breaker Power that you only get one mandatory use out of it to progress in the main game (and three overall), but it's very much worth it.