Breather Level: The second level of Circus Park, compared to the rest of Circus Park, is easier; there's not as much hard platforming that made the first level of Circus Park hard, and it mainly has straighforward fighting enemies in it. It's also ridiculous that the second level also has the game's easiest boss in it.
Likewise, the third level of Blizzard Castle (where the objective is to make the head of a snowman) is relatively simple, takes place in a small area, and doesn't feature many enemies. It comes as a welcome relief after Moles.
Having trouble with he Red Rail puzzles? Just jump with an egg a little bit into the rail and you can ride it like a normal Blue Rail.
Having a hard time cleaning up the puddles of darkness to allow yourself to damage Dark Raven? Don't worry, there's an alternate way to stun him: When he fires some dark magic missiles at you, just guide them back at him and hit him with them, and you'll stun him as if you cleared a pool of darkness.
Fridge Logic: The crows, diurnal birds with horrible night vision, want to bring about a world of eternal darkness?
Nightmare Fuel: The final boss battle music is basically a darker version of the main theme. What really sets it into this territory is that the children that sing the main theme are now ominously chanting in this one.
Billy can't do anything athletic without an egg. Crushing enemies under your giant egg is satisfying and fun. Jumping with the giant egg is anything but fun. Jumping to higher ledges is... unpredictable and infuriating. Bearing in mind Billy Hatcher is a platforming game, not a boss-run, so the problems quickly start to mount up. Whenever you 'egg bounce' up to a higher platform, Billy almost always falls off, leaving the egg now stuck up in a place you can no longer get to. The player is now forced to either ditch it and search for another one... or if it's the only egg available... wait for it to disappear and re-spawn. This is doubly annoying if you had already fed some fruit to that egg, and triply annoying if you wanted to hatch it.
Tastes Like Diabetes: Billy Hatcher is clearly aiming towards a much younger demographic than most of Sega's intellectual properties, which has regrettably alienated a lot of older gamers from ever picking it up.
That One Attack: Most of Dark Corvo's attacks aren't too tricky to dodge, but when he summons the Saltim shadow clone and when Dark Corvo clones himself and attacks you is when it really gets difficult.
That One Boss: Moles the walrus. He first starts by ice skidding towards you, which is easy to dodge, but then he rolls into a sphere of ice, and very rapidly starts bouncing around the room, and this attack is very unpredictable. Also, to make matters worse, every time he hits a wall, small ice spheres appear and start bouncing around the room with him. Eventually, he will stop and allow you to hurt him, but those ice spheres are still bouncing around, and while you're focusing on hurting him, those ice spheres can hit you, stunning you for a second, and he will repeat this process for the whole fight.
Dark Corvo the crow sorcerer is even worse; a lot of his attacks are extremely hard to dodge. For example, his shadow clone of Glur shoots homing bouncy balls, and his Saltim clone rapid-fires bombs at you, which are very hard to dodge; also, when Dark Corvo clones himself and attacks you, he attacks at high speed, and there is no clue as to which is the real one, and if you don't hit the right one fast enough, he will repeat the attack. It is especially painful that Dark Corvo comes after one of the hardest levels in the game. He's so hard that the game actually warns you he's gonna be hard, and this is a hard game!
That One Level: Mission 1 of Circus Park... Just wow, this stage can seriously drive you mad. For those who don't know, you have to hatch a Golden Egg (which will be destroyed in one hit from a crow attack/hazard), through a narrow area with fire, red rails, spinning, and giant cat crows with very little fruits to grow the egg without killing said crows. Oh, and to make matters worse, the Egg will usually FALL THROUGH THE RAILS IF YOU TRY TO DO THE LEVEL LEGITIMATELY. The only saving grace is that you can bypass the glitchy rails by riding them like a blue one via jumping onto them, but still.
Toy Ship: It seems a decent amount of people in the fandom like the idea of Billy and Rolly. Sadly, the only thing that gave any kind of Ship Tease was a 2-second clip in the opening cinematic.