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.
Angst? What Angst?: The most Captain Viridian gets sad is when he loses his crew, and that's only for the first five minutes of the game. Later, when he destabilizes the dimension, he is momentarily alarmed... but then he quickly chippers up and runs through the funky-colored rooms as if nothing happened.
It's Short, so It Sucks : Probably the complaint made most by reviewers about the game, especially given its "high" initial retail price of $15 - depending on skill, a blind run-through takes around two to four hours. Price drops and extra content have nullified this complaint to an extent.
Obtaining one of the trinkets requires you to go through about 6 consecutive screens filled with nothing but spikes, which on its own is extremely difficult. Then the game requires you to turn around immediately (or get impaled) and complete the same six screens in reverse order. More than one player has missed the (rather easy) landing at the end, after successfully navigating Hell itself...
Lampshaded in that there is a message at the beginning stating "Ha! Nobody will ever get this one."
Several of the trinkets. Veni Vidi Vici is the most infamous, but Prize For the Reckless and Edge Games are also candidates. Thankfully, these are optional.
Not optional, however, is the Gravitron. 60 seconds of crazy, confusing dodging.
"...Not As I Do" has a trick you need to figure out in order to complete it, but even once you figure out how to do it, you still have to do it, and the timing and back-and-forth motions required make this level hard enough to...let's just say there's at least one Troper who's still stuck on that level, having never seen the rest of the game due to it.
The Gravitron is this in No Death Mode, since the checkpoints every 5 seconds are also gone. Normally, being hit also clears the screen of all the obstacles while knocking the timer back to the most recent multiple of 5 seconds, giving you a brief lull during which you have a chance to reposition yourself and prepare for the upcoming wave. But since getting hit once in No Death Mode ends the game...
The "A New Dimension" level featured in the 2.0 update has "Remember Me?". It's "Veni, Vidi, Vici", made to be harder... and mandatory.
Souleye's other bonus level, "The Dual Challenge", is no slouch, either. "Cetology Gone Wrong" requires incredibly precise jumps, and before that is a five-room shaft similar to Veni Vidi Vici except (thankfully) only going one way but you have much less room to maneuver. Also, one part of the shaft has a gravity inverter and hitting it pretty much spells death.