Bottomless Pits / Super Drowning Skills: All pits are large pools of water. And well, it's a platformer, touch the water and you die. But you have infinite lives! Yay!
Bubbly Clouds: After your first eversion, those clouds are nifty platforms!
Easter Egg: In the newest version (as well as the HD), you get a super awesome third ending if you end Level 8 on the awesome X-4 and be sure to collect ALL the gems FIRST! You can also unlock a super awesome bonus title screen if you 100% the game!
Follow the Money: In the first stage already, you can see some gems just out of reach above the clouds. They're hinting that you have a special ability which helps you to reach these gems.
Game Over: Averted in regular gameplay, since you have unlimited lives, but after you reach the princess, the game is nice enough to close itself.
Genre Busting: It's a platformer AND puzzle game, with some neat surprises thrown in too!
Ghibli Hills: Look at the green grass and blue skies shown in the page image! Isn't it a beautiful place? The HD version also puts nice hills in the background!
Happy Ending: The good ending, where the flower man finds the princess and they're so happy to be with each other! Aw, lookit the heart in the background <3
Let's Play: By DeceasedCrab. It's also been LPed blind by a fine fellow from the Let's Play Forum. It's actually one of the most let's played freeware games of 2009.
Multiple Endings: A bad one at the end of X-7 if you don't have all the gems, where Zee Tee fails to rescue Nehema. But when you collect all of the gems and beat X-8, you get the Good Ending where Zee Tee and Nehema live Happily Ever After! The HD version even has an extra special ending where Zee Tee and Nehema are together in pure happiness forever and ever!
Sweet Dreams Fuel: Nearly everything, but the first three or four levels, and possibly the eighth are certainly the most memorable!
Take That: The 'Metaphor for the Atomic Bomb' achievement takes a jab at another Mario-esque platformer with an added mechanic. The tone is evident from the icon for the achievement: Zee Tee and Nehema making the 'awesome' face.
Unwinnable by Mistake: Some level designs can lead to the player getting stuck in one place without being able to get out or die, and the level must be restarted. Some instances of this were corrected in later versions, but it's still possible to purposely enter a unwinnable state in some locations. Fortunately, you can quit the level and restart without losing any of the gems you've collected during that level!