A minor example, as it is limited to the demo version, but the Project Mirai Deluxe demo could only be played once in Europe, while the American version can be played 30 times. Later subverted when SEGA admitted that this wasn't intentional and that players who want to play the demo more than once should contact the eShop customer service.
The exclusion of English translations in certain games, including F, DX, and Future Tone, although it's more of a nitpick than anything else. DX gets more flak for this than the other games; F simply didn't get licensing for English subs at the time because it was the first localized game, and Future Tonesimply has too many songs.DX doesn't get as much of a pass for overseas fans because it came out after F 2nd, which did have English subs.
You wouldn't notice it unless you compared the Japanese and international versions side by side, but the localization of DX cut out all of the voice clips except for the one where they tell you the date and time.
Colbert Bump: General rule of thumb is that if a song with relatively little popularity makes it into a Project DIVA game, expect it to experience a popularity spike.
The polar bear and panda with heart-shaped ears in the PV for "Love-Hate" are mascots used by HoneyWorks' composers Gom/FernandoP and Shito/Chorisu-P. mirai DX's "Terekakushi Shishunki" also has an indirect cameo with Len's and Kaito's outfits, which are based on the panda and polar bear mascots respectively and double as a reference to the original PV.
40mP and his wife both appear briefly in the PV for "Do Re Mi Fa Rondo", using the same illustration as in the original video.
Pinocchio-P's characters Aimaina-chan and Doshite-chan, who he frequently uses as mascots, appear in the PVs for "Arifureta Sekai Seifuku" and "Slow Motion," as well as the opening movie of mirai DX, which uses a theme song he composed. Aimaina-chan is also a purchasable room decoration in DX.
Early-Bird Cameo: In Project Diva on PSP, Hello Planet has a mini-game version based on the non-extend music video. This mini-game can unlock a second, alternative ending for the music video it's based on. As of the end of 2015, it has not appeared in any game with an alternate PV.
Easter Egg: In Project DIVA Arcade Future Tone, and likewise its PS4 port, there's a song called "Master of Puppets." The logo for the song is a direct reference to Metallica, a band that made a song of the same name.
Edited for Syndication: Very often, songs are pared down in length to keep them within the 3 to 4 minute mark so levels don't drag on too much. This even applies to the theme songs. Strangely inverted for "Poppippo!", which was too short and had to have its length doubled by adding the English version.
This actually poses a bit of a problem in X regarding "Name of the Sin", in that the lyrics that are most vital to the song's story are cut. This leads to a rather jarring moment when the singer of the song suddenly starts talking about someone she fell in love with and a gift from "him" despite never describing who the person actually is - to the point where the official translation actually mistranslates one of the lines.
mirai does not do this to any of its songs, even songs that were also in DIVA. This has the unfortunate side effect of creating Marathon Levels with unnaturally long tracks like Reverse Rainbow. Songs that wind up being ported back to the main series via Arcade usually undergo this process, like "Piano Girl".
X has Festival Mode, which allows you to combine three songs onto a single setlist and play them like you would a Medley. This trope takes effect to keep the total length below limit as opposed to making you play three full songs in a row.
The English-subbed version of the "Together with Mikudayo!" series. Whereas the original version had five episodes, the English version had three. The third English episode is episodes 3, 4, and 5 spliced together.
2013 saw the release of Megpoid the Music#, a similar rhythm game for the PSP featuring the popular Vocaloid GUMI.
Gumi also appears in selected songs in Project mirai, however she has not been licensed for Project Diva itself.
And in 2014, IA/VT Colorful does the same for IA on the Vita, courtesy of Marvelous AQL.
Miracle Girls Festival is essentially Project DIVA, but as a Massive Multiplayer Crossover with several anime franchises. Especially notable in that Miracle Girls is also made by SEGA, runs on the exact same engine as DIVA, and features very similar gameplay.
Persona 4: Dancing All Night is a similar situation, except it goes one better, with the character designer using a module he'd designed for Miku to have her make a cameo... which was followed up several months later by that module being officially added to Miku's Project Diva modules in every current game released at that time.
The PlayStation Store description for Luka's Nagisa Replica in F 2nd conveniently omits the fact that the outfit is from specifically Phantasy Star Portable 2Infinity, when only the original game was localized.
The music video for "The MMORPG Addict's Anthem" in Project Diva F takes place (and debuted) in Phantasy Star Online 2 and features Miku dressed as a FOnewearl. Project Diva F was released overseas in 2013, while PSO2's North American localization remains in Development Hell to this day.
The depressing thing being here that Phantasy Star Online 2 was known to have a english translation in 2012, with heavy promotion at the Penny Arcade Expo that year. SEGA apparently put it on indefinite hiatus due to the use of premium currency, a fact that killed Phantasy Star Universe on XBOX 360. note Embarrassingly, Sony, unlike Microsoft, HAPPILY support ARKS Cash on the PS4 and Vita versions, and Google will happily convert dollars into yen 1:1 for the mobile version, and it's DIFFERENT premium currency issues which are blocking it!
One extreme case of this happens with GUMI. She appears the generic 'visitor' on the Mirai games, and sings in several songs, but has appeared in NO other games released worldwide.
Also, anyone who has only seen the concerts would be forgiven for thinking that Akita Neru, Yowane Haku, Sakine Meiko, Kasane Teto and Gumi were made specifically for the games.
Milestone Celebration: In honor of Hatsune Miku's 10th birthday, a special DLC pack was put together for Future Tone that includes a song that hasn't appeared in either Arcade or any other game in the series: DECO*27's "Ghost Rule". It also comes with a Future Tone-exclusive anniversary Module.
No Export for You: The games that haven't been sent overseas include DIVA, 2nd, extend, mirai and Arcade. The last one can technically be considered a weird exception as well, since Round 1, a Japanese amusement center, has branches in the United States that host Arcade cabinets.
Notably, Future Tone on PS4 subverts the last one outright, being a complete port of the Arcade game with some Arcade-specific features removed, note like the credits system, and the first three by token of that virtually all the songs from Diva, 2nd and Extend not already in F 2nd, X or Mirai DX are in Future Tone itself. mirai was subverted by an updated rerelease of the second game coming west around the same time.
For an unusual level-specific example, "PoPiPo ~Special Edition~" is the only piece of F content that never made it outside of Japan.
Official Fan-Submitted Content: Piapro often accepts fan-submitted artwork through their website that eventually becomes the series' loading screens. A good number of modules also come from Piapro collaborations.
A number of songs that were ported to Arcade have fan-submitted videos that were chosen via a contest, where players created a new PV through the home console version's edit mode. note ATLUS producer Sam Mullen confirmed thishimself when talking about Future Tone over Twitter, and that these alternate fanmade PVs remain in FT.
Recursive Adaptation: The video for "Black★Rock Shooter" appears primarily inspired by the anime Black★Rock Shooter than the original PV itself (down to BRS's signature blue flame in her left eye and Black Rock Shooter's cameo), as well as the song being the version from the 2012 anime rather than the original. So it winds up being that Project DIVA F channeled the Black★Rock Shooter anime, which channeled the OVA, which channeled the original song.
Remade for the Export: Mirai DX was originally intended purely for non-Japanese audiences, being effectively just mirai 2 with one extra song and a few bonuses. Japanese fan demand for the Regional Bonuses caused it to be released in Japan as well (and, ironically, still before the English release, due to the latter getting delayed). Predictably, it didn't sell very well there, due to being little more than an Updated Re-release.