Demonic Spiders: Those Wario heads in the final level. They are incredibly annoying to fight and cannot be skipped. The final boss is just up ahead, and you do not want to lose your preferred power-up before you face him. Unfortunately these things bounce all over the place making them very difficult to predict, take multiple-hits and have an invincibility frame of their own, causing you to take damage from an ill-timed jump! You cannot be blamed for using the 100-enemy-star trick to defeat them.
Even Better Sequel: A longer, more elaborate, and all in all much better game than the first Super Mario Land.
Game Breaker: The Bunny Ears. You can float infinitely, descend slowly, and jump higher, but the lack of ability to spin jump (That the Super and Fire forms can do) causes some to call this a Power Up Letdown than a truly useful power.
Star Maze. Those stars are going to kill you many, many times.
While it's expected that a final level would be difficult, Wario/Mario's Castle can be absolutely brutal. While the rest of the game (barring Space Zone) is relatively easy, the Castle is a difficulty spike that can be very jarring for first time players. Combine pools of acid/lava, swinging spiked balls, spiked traps, and several jumps that require almost-pixel-perfect placement, and you've got a tough level. And no checkpoints either. On the plus side, the Bunny Ears power-up makes the stage much easier to manage, provided the player is able to keep hold of it.