Dem Bones: Gray Fairies, standing in for Dry Bones.
Easter Egg: If you have a mushroom in reserve when you face Alice, throw it out at her and watch what happens. It's a One-Hit Kill to her, referencing the fact in the prequel, all mushrooms are hazardous to her.
Improvised Platform: Tenshi Suit Marisa can create keystones as makeshift platforms. Even better, jumping from them will cause the stone to fall, taking out an unlucky enemy that happens to be standing below.
Nintendo Hard: Not as much, but the game can be brutal when it wants to be.
Not Completely Useless: Suwako Suit Marisa is more functional in water levels (and there aren't many of them). On land, all she can do is hop, so it can be rather difficult to aim your jumps. On the slight plus side your actual jumps will be higher than normal.
The Yukkuri powerup despite its drawbacks, is invincible while rolling and rolling is sometimes the prefered way of getting around a level.
Point of No Return: After getting the last mushroom from Nue, you will be immediately sent into the final level with no other option. Best to stock up beforehand because you will not be able to revisit shops.
Power-Up: The Reimu Suit, Reisen Suit, Suwako Suit, Tenshi Suit, Super Marisa Mushroom, Yukkuri Mushroom and UFO. The last two could qualify as Poison Mushrooms, outside specific situations...
Replay Value: You can replay certain levels to find hidden exits.
Sequence Breaking: The secret exits in stages 1-4 and 3-5 allow you to skip to Worlds 3 and 7, respectively. Upon defeating the World 7 boss, you go right to the final area, without needing to collect the other mushrooms.