While the franchise has been relatively obscure in America for some time (but really big in Japan) outside of Memetic Mutation, the new 2012 anime, as well as its immensely positive reception from many anime reviewers on the Internet, has caused its popularity to skyrocket stateside. Its popularity in English-speaking territories further improved when both the 2012 anime and the Stardust Crusaders sequel were added to Crunchyroll.
The 2012 anime became one of these for the band Yes, due to usage of their most popular song, "Roundabout", as the anime's ending theme song.
For some reason (at least in Japanese), and until the TV series came out, most of the characters were voiced by older voice actors, despite voicing characters who are supposed to be teenagers or young adults. This is more exaggerated in the Part 3's OVAs when almost everybody are voiced by people who doesn't even try to sound like young men (excluding Joseph, for obvious reasons).
Young Jonathan and Young Dio in Part 1 are both voiced by the same actors who voice their adult selves. They also don't make an effort to sound like adolescents.
Inverted with both Erina Pendleton and Suzi Q, in the TV series' Parts 2 & 3: Both VAs (Ayako Kawasumi and Sachiko Kojima, respectively) are normally typecasted for voicing younger girls or young adults, but they still voice their older versions in Parts 2 and 3, rather than casting much older VAs for the job.
The same goes for both Speedwagon and Straizo's voice actors, since they are voiced by the same ones who voiced their younger counterparts in Part 1.
Part 8 JoJolion has gained the nickname "Four Ball Run" due to the protagonist's...unique anatomical arrangement. The main character himself is also often referred to as Gappy or Jo2uke to avoid confusion with the protagonist of Part 4 in English-speaking fandom.
"Holy Wubstorm" (after Wamuu's special attack Holy Sandstorm) for the Pillar Men's dubstep-flavored Leitmotif in the anime.
Spoilers for Part 7 ahead. Lucy Steel's temporary Stand is known as "Love Train"; however it appears in chapters called "Ticket to Ride" and was later confirmed to be named as such in JOJOVELLER.
Pocoloco's Stand is sometimes called Buckethead due to the shape of its head.
Keep Circulating the Tapes: A film adaptation of Phantom Blood came out in theaters in 2007. It hasn't seen any sort of home release. However, an incomplete cut (that is, there is no voice acting, and occasionally full animation is replaced by storyboard stills) of the first 16 minutes has appeared on the internet.
No Export for You: The manga aside from Stardust Crusaders for America, one SNES RPG, and two beat 'em up games for the PS2.
This gets averted twice, since not only the Animation (2012, and "Stardust Crusaders") is being streamed on Crunchyroll since July 2014 (with very few edits or name changes), but Viz will also publish Phantom Blood (September 2014) and Battle Tendency (March 2015), both based on the current "Jojonium" Japanese release of the series, with future parts possibly following if sales hold up.
Troubled Production: Apparently, one of the reasons for the poor-quality of the Phantom Blood movie and why it's never been released on DVD was because one of the producers was killed in an accident and his family pulled the plug on funding, leaving an incomplete, rushed mess of a film.
And there's a 16-year-old who has a 30-year-old nephew because his father was in his fifties when he was conceived. That's before we even get into where Giorno Giovanna fits into the Joestar family: Giorno is Dio's son, conceived after Dio stole Jonathan's body, which makes Jonathan Giorno's biological father. This makes Giorno Joseph's uncle. Note that Joseph is in his eighties by the time Part 5 starts and Giorno is fifteen. No series can top this one when it comes to sheer WTFness.
By sheer nature of his never having been human, no villain can top Kars for WTFness. (Except perhaps an out-of-it Pucci in one particular chapter.) Of course, this also applies to just about any other series as well. The stuff he could do, simply unreal!
But the most bizarre thing is the mangaka himself — he hasn't aged a day in over 20 years (either that, or he's aging backwards). Recent photographs show him looking younger than he did when he started on Phantom Blood...back in 1986.
Tying in somewhat to the first bit of trivia, was the very first scene depicted. The story actually started before Jonathan's time, back at least two millennia in some South American civilization, which eventually died out for one reason or another, had used the Stone Mask to gain immortality as a vampire. And the first character shown was a maiden to be sacrificed. This lost tribe is later revealed as the Pillar Men, though it was specifically Kars who created the Masks.
The phrase "Lucky Land" appears frequently on signs throughout the series, a reference to Lucky Land Communications, the company Araki formed to formally hold copyrights for the series.