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.
Disappointing Last Level: Many players feel this way about some of the boss songs, feeling that they were designed more to fit their respective charts with less regard for track quality. "Thor's Hammer", "DIAVOLO", and "Elemental Creation" are a few such examples.
The final unlock of tricoro's "Our War of the Worlds" event, which appears to be the game's final unlock event, runs on a lifebar that is damaged every time the player plays through the game. Fortunately, the lifebar was shared by all players on the network. Unfortunately, not only was the unlock tedious, taking many days to unlock even just the Normal chart, but the unlock song itself, "Kyatorare Koi wa Mo~moku", is a cartoony "denpa" song that many players consider to be highly annoying, or at the least, not fitting as the Final Boss song of tricoro.
There's actually a bit of a running joke amongst certain IIDX communities in both Japan and the west that Bemani and Pepsi (and occasionally other soft drink manufacturers) conspire to create new color-coded flavors for each installment. Every subtitled style has a flavor of Pepsi (or Mountain Dew in some cases) that seems literally made for each other; it has extended to other Bemani franchises on occasion, such as Pop'n Music with Carnival-flavored Pepsi...
DJ Yoshitaka's remix of "Ghostbusters" retains the original English lyrics, but with hilariously bad pronunciation. Foo you gone car?
"Poodle" due to its lyrics, which are in hilariously broken English and are displayed karaoke-style in the video. A sample:
Walking with you on a happy sunny day you know me clever more than retriever Washing me everyday tidy neat you know me precious dog as a dynamite Believe in me If it's rainy again Nobody wanna cry Nice cow bone you are my lover
Newer Than They Think: A number of songs, most famously "A", "AA" and any of Osamu Kubota's songs, despite sounding classical, were composed specifically for the series.
Just try playing "VANESSA" with the portrait in the background "video" staring at you. It was replaced by a generic video in tricoro, for better or for worse, only to be put back in PENDUAL.
The background video for "INORI", consisting of robotic abominations with red singular eyes and a faceless doll who runs away from them and jumps to her death.
Sequelitis: 9th Style, the first IIDX game to use Windows-based hardware, is riddled with timing problems, as well as a Game-Breaking Bug in which attempting to play the song "quasar" will sometimes crash the game. However, these issues appear to have been ironed out in 10th Style.
Polished Port: 9th Style on PS2, which not only looks prettier than its predecessors and runs at a higher resolution, but also doesn't suffer from the same timing problems that its arcade counterpart is infamous for.
Don't you dare admit that you use the Random modifier on scale-heavy charts. Strangely enough, the opposite applies for all other charts once you're at the level 10-and-up range: Don't you dare admit that you don't use Random.
Using Assist modifiers and even just having the purple Assist Clear lamps on charts tends to draw mockery from some players.
Tastes Like Diabetes: Many of the more pop-ish songs in the franchise will inevitably have videos with this trait. Empress as a whole could also count as this, although thankfully it wasn't as bad as many thought it would be.
That One Attack: "CONTRACT" [Normal] is generally around the difficulty of a level 7 chart...right until the very end which throws a series of mini-scales combined with scratches. It is ONLY because of this part that the chart is rated a 9, making it one of the three highest-rated Normal charts (the other being "Sense 2007"'s and "gigadelic"'s).
For those clearing out level 9 songs, "Watch Out Pt. 2" is particularly frustrating, thanks in no small part to the abundance of scratches.
"THE SAFARI", notorious for dashing the hopes of those trying to achieve 7th Dan and having been the final song of the 7th Dan course for many versions now. It's to the point where some players will just skip straight to 8th Dan, finding even "gigadelic" on Hyper (which by the way is rated a 12note more due to the ending than anything else) to be easier than THE SAFARI.
"G2" on Normal is no slouch for a level 8 due to the masses of drumrolls near the end of the chart; there are plenty of level 9 songs that are easier to clear. Did we mention this is a level 8 Normal chart? Normal charts typically peak at level 7.
"CONTRACT" on Normal is rated a 9. For the first 97% of the song, the chart doesn't seem to warrant its level-9 designation, with its eighth-note scales posing next to no threat for players who can clear 8's and 9's, save for a suspicious-looking pattern at measures 48 and 49, roughly 2/3 into the song. But then comesthe very last two measures, which suddenly throw sixteenth-note scales and drumrolls combined with scratches. As a fine example of the pass/fail-score dissonance, there are players who have achieved an A or even AA grade on this chart and STILL fail it.
"gigadelic" is a pain in the ass no matter what chart you play. Even on Normal, the chart has a few annoying drumroll sections, and an ending with repeated uses of the 7 key and the turntable, and of course a Last NoteDifficulty Spike. It's also rated a 9, making it one of the hardest Normal charts in the entire series. Its Hyper chart is rated a 12 simply because of the ridiculous ending, and has killed many potential 8th Dan players, being the final stage of the 8th Dan course in every arcade IIDX version since IIDX RED.
Most of DJ Mass MAD Izm*'s songs, which are notorious for having extremely high scratch counts, making them extremely difficult to time, especially if mixed in with key notes as well. In SPADA, "Shakunetsu Beach Side Bunny"'s Single Another chart, well-known for a record-breaking scratch count of 667, was introduced into the Kaiden course, to the agony of many top-tier players.
A word of advice to those playing on Double: Avoid most Double charts produced before GOLD. Many early Double charts were designed with 2-player non-Battle play in mind, resulting in charts that are extremely awkward to play for one player. A sterling example of this is Cheer Train (DPA); the song itself is light and cheerful, but the chart is rated a 12 because the chart expects you to be able to frequently hit notes and scratches at once on the same side. Unless you use the Random modifier and are particularly lucky, this chart is humanly Unwinnable by Mistake.