Some of mine:
4 pillers at Shifting Sand Lands: I really had no clue what to do. Plus I always died trying to get to the pillar near the blocks and quicksand. I never ground-pounded my way down so yeah.
Wall at Whomp Fortress: Seriously how the hell are you supposed to find this thing. I'm just assuming that everyone found this one out by horsing around.
Toad in the Hazy Mazy Cave room: This was my last star, as lame as it sounds. I had no idea he was in that room...
4 pillers at Shifting Sand Lands: I really had no clue what to do. Plus I always died trying to get to the pillar near the blocks and quicksand. I never ground-pounded my way down so yeah.
Wall at Whomp Fortress: Seriously how the hell are you supposed to find this thing. I'm just assuming that everyone found this one out by horsing around.
Toad in the Hazy Mazy Cave room: This was my last star, as lame as it sounds. I had no idea he was in that room...