These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Breather Boss: Because King Knight and Specter Knight don't gain any new moves during their rematches, they're relatively simple compared to the other members of the Order.
Propeller Knight and Tinker Knight are challenging but simpler than previous bosses and Polar Knight.
Demonic Spiders: The Green Propeller mooks introduced in Propeller Knight's stage. They LOVE to push you into Bottomless Pits every time they appear and these enemies are rather fast and difficult to deal with since you'll have to watch your footing while dealing with them. They will make you rage in the Tower of Fate.
Everybody loves the Propeller Rats! The Clockwork Rats and Plague Rats later in the game are just as cute if a bit more threatening!
The wolves on the Stranded Ship became a huge hit. It helps that they have a cute design, howl, and have just as much trouble on the ice blocks as you do.
Faux Symbolism: King Knight can stun Shovel Knight with one of his attacks which will make him kneel, what he does next? Bash him with his scepter. He's "Knighting" him.
Fridge Brilliance: Why one would use a shovel as weapon? Because not only it is a weapon, it is also useful to dig up treasure! It also helps to remind that Shovel Knight was previously an adventurer and, quite possibly, a treasure hunter.
Game Breaker: The Phase Locket found in the Lich Yard. It's one of the first magic items you can acquire and grants two seconds of invincibility. Combine that with a low use cost and the fact that most enemies drop magic refills on death and you can cheese your way through otherwise difficult fights and puzzles.
Seems to be fixed now- the cost has been increased, making it much less game breaking, especially if your magic hasn't been upgraded very far.
Most Wonderful Sound: The little jingle that plays whenever Shovel Knight scrapes a chest empty or find a music sheet.
The *shing* sound made by Shield Knight when she's offering her help to attack The Enchantress or to protect you from her all-screen attack.
That One Achievement: Perfect Platformer and Impossible, the first requires you not fall even once in a Bottomless Pit, the later not to die once. Thankfully the game doesn't record your deaths if you simply leave the stage to the world map.
Hurry Up! which requires the player to finish the game within 1h 30m, screw a few times and you might as well restart the entire game again. Even returning to the map can become problematic since every time you do so the game will add few seconds to the total time which will accumulate if you restart one too many times.
Dirt Poor. It requires you to get absolutely no gold during a run through a member of the Order of No Quarter which is harder than it sounds. You may inadvertently pick anything work a single gold after breaking blocks of dirt of killing an enemy and there are several stages where getting this feat is impossible as there are gold worth gems right in places where you must pass.
True Shovelry. To earn it you must beat all levels not getting any relic at all, which means absolutely no recovery other than food, no cheesing out victories with Phase Locket and no sequence breaking with Propeller Dagger.
That One Boss: Polar Knight is the only member of the Order that can instant kill a player by summoning spikes. He also blocks against downward strikes.
Propeller Knight when fought in his airship. He will break parts of the arena and try to pull or push you in a Bottom Less Pit for an instant kill, he also can counter shovel down strikes by blowing Shovel Knight very high and try to impale him with his sword.
Clockwork Tower, Tinkerer Knight's stage. The final segment of the stage will auto scroll upwards meaning it will constantly create Bottomless Pits, one mishap with the jump and it's back to the last checkpoint with a hit in your wallet.
Tower of Fate: Entrance. Difficult jumps? Check. Demonic Spiders? Check. Difficult boss? Check. This stage is regarded as one of the most difficult by players for a reason. Trying to finish it without relics is almost self flagellation.
Tower of Fate: Ascent. It has a segment where you must push a statue to create bridges all over a huge Bottomless Pit. The catch? Enemies positioned specifically to throw you off the said statue and two green mooks from Propeller Knight's stage to downright blow you into the Bottomless Pit. It also has an auto scrolling section like Tinker Knight's stage, now with blocks moving around to crush you as well.
Even more so in New Game+. The ONLY checkpoint in the level is the one right before the Boss Rush. Said boss rush is now even more difficult, as the you take double damage and full health turkeys come only after every OTHER fight.
Though it so happens that the aforementioned statue section can actually be skipped entirely by getting the statue above the obstacles and just jumping above them (requires usage of the Propeller Dagger to clear some of the jumps, though).
Wake-Up Call Boss: Specter Knight is a good deal more challenging than King Knight and requires players to learn his more complex attacks.