Cody's theme in SSFIV is heavily based off of the intro from Final Fight. The rapper repeats the words "Turn the beat back!" quite often. This is a look into Cody'spsyche; he longs to relive his glory days as a hero and get out of his funk, but sadly has convinced himself that things can never go back to how they were.
Near the end, the song gets rather uplifting and whimsical... until it loops and goes back to being a gritty rap. More than likely, it's Cody remembering his time as a hero, only to go back to his current, uncaring self.
As of SSFV, Cody's theme has been given a more upbeat remix, signifying Cody regaining some of his old personality as mayor of Metro City even though he still doesn't regard himself to be the hero he used to be.
Kinda doesn't work anymore now that Makoto has risen quite high in the tier lists thanks to the buffs she got in Arcade Edition and Arcade Edition ver. 2012.
Makoto has unfortunately dropped back down to low/bottom with Ultra (currently 36/44 as of this writing), so this one's back in play, though she's at least still higher than Dan.
One aspect of the US marketing was slapping on surnames to some characters, and Ryu became Ryu Hoshi. Factor in the Japanese name order, and it ought to be Hoshi Ryu. And hoshi means "star" so the full name in kanji can also be read as Seiryu - one of The Four Gods.
Couple this with the fact that his stage in every iteration of Street Fighter II is the Suzaku Castle and you can see a motif coming. Where Byakko and Genbu come in is something else (though Byakko can be argued to be Sagat, considering his own "tiger" motif, and Ryu apparently takes more self-improvement from his rivalry with him than from the one with Ken).
Dan is significantly less wimpy in Street Fighter IV than in previous appearances. You might think this is because the developers are just trying to make him more fun to play but think about what Dan has been doing for the past three years. He's been hanging around with Blanka and Sakura. Even if he is getting his butt kicked regularly, practice with both fighters is bound to improve his fighting style.
Juri hates Shadaloo for killing her parents and maiming her. S.I.N. is stated to have been Shadaloo's weapons division. In her Story Mode, Juri joins S.I.N. and works for its leader, Seth. Is it that she has/had no clue of the link between Shadaloo and S.I.N.? Well, if you examine her ending, it can very easily be a case of her playing the role of The Chessmaster, playing both sides against each other to make the whole organization destroy itself from within—that would be her ultimate revenge!
Seth's was named after former Capcom employee Seth Killian and is also derived from the Biblical Seth (Being that Street Fighter's Seth replaced Abel as Shadaloo's main experimental subject after the latter fled). Seth's name also coincidentally could have a third interesting level of meaning in referencing the Egyptian god Set (also known as Seth), which fits with his desire to become a god-like being.
Why does Dan's Japanese voice sounds so husky and gritty in SFIV and its subsequent sequels? Well, since Osamu Hosoi voiced him from the Alpha series up until he was replaced by Toshikyuki Kusuda, it could be a sign that constantly screaming "OYAJI!!!" for years eventually took a toll on Dan's vocal chords.
In Street Fighter IV, whenever a Shoryuken is performed, it has a certain "tail" following it, either dark black like ink, transparent like air, flaming, or white. These all correspond to the force used by the person performing the Shoryuken. Ryu, Akuma, Evil Ryu, and Oni's Shoryukens all have an ink-black tail because they use the Satsui no Hado in various quantities to power it. Ken's Shoryuken is flaming because Ken is channeling his aggression into it. Gouken's Shoryuken is pure white because he uses Mu no Hado, and Seth's Shoryuken has a transparent tail because it is "empty," and isn't using Mu no Hado or Satsui no Hado.
The reason that Dan is still incredibly weak compared to the other fighters is because, according to Gouken, he refuses to let go of his resentment for Sagat killing his father Go Hibiki. At first it might not seem like this would make much of a difference, until you remember that Sagat wasn't very strong himself (Gen didn't see him as worth his time) until he let go of his resentment for Ryu scarring him in the original World Warrior tournament and they became friends in Alpha. Now Sagat is so powerful that Oni considers him a Worthy Opponent. Makes you wonder what Dan could be like if he could let his anger go.
Adding onto this, Chun-Li once fought Bison in A Shadow Falls and lost due to her overwhelming desire for revenge, recklessly running into an empowered Bison. Akuma even calls her out on this in IV, scolding her because of being preoccupied with the dead. Dan never really forgave Sagat due to him not getting over him (Sagat) killing his father. In III, Chun-Li has finally found peace and has finally got over her father's death due to Shadaloo's fall. Another reason why Dan never got so strong and instead got reckless was because he still holds on to what happened in the past, even if it was an Accidental Murder. Dan never really found peace after what happened.
Dan's win quote against Sagat has him proudly shout that he finally beat Sagat again (even though this may never happened in main storyline) Dan still had some hatred over Sagat. Even Sagat's win quotes against Dan hints that Dan may have found out that Sagat threw the fight or may have tried to beat Sagat one more time and even reveals that Dan is still bitter over his father's death.
M. Bison being voiced by none other than Norio Wakamoto makes sense because both are Evil Is Hammy, both are cultural icons in their respective fields (Norio being a famous voice actor while M. Bison is known for being a Magnificent Bastard villain responsible for screwing with pretty much every SF character ever), and not only that, both are also deadly martial artists!
It's been known for a while that Vega and Balrog don't particularly like one another (Vega seeing Balrog as an unrefined, idiotic brute, Balrog thinking Vega is a flamboyant, weakling pretty boy), to the point that, in Street Fighter X Tekken, their ending has the two about to partake in a Duel to the Death thanks to Pandora. Even with the Dub Name Change, Balrog represents the bison/bull/buffalo (if his Japanese name and the Theme Naming present in his moveset are anything to go by). Vega is—when keeping up his appearances—a matador, so it's possible he sees Balrog as an anthropomorphic version of the wild beasts he tangles with day-in and day-out. Given how obsessed Vega is with the concept of beauty, you can probably see why this is a problem.
It makes sense why (outside of real life reasons) Akuma wasn't in Street Fighter V initially ; he was working as Kazumi Mishima's hitman in Tekken 7. And now that Fated Retribution has been out on arcades for some time, Akuma, his role as a guest character fulfilled, has decided to come back as DLC.
Charlie's theme in Street Fighter V starts with a piano riff that sounds suspiciously like the one that plays in his ending in Alpha 2. This provides a subtle Foreshadowing to the fact that in Charlie's story in Street Fighter V, Charlie's Alpha 2 ending is confirmed to be canon.
F.A.N.G is very kooky and crazed in gameplay but is cold and calculating outside of battle. This seems odd, but remember his techniques depend on his body producing and resisting poison. He'll have to produce more when he's in a fight. Perhaps he's not totally immune to his own poison, and it affects his mind a bit when he has to make large quantities of it.
Also, his boast that he'll defeat his opponent in two minutes sounds weird until you think about his fighting style. As a poison-wielding assassin, his fighting style revolves around injecting opponents with poison to kill opponents as quickly as possible. In other words, he actively tries to kill in under two minutes.
Next, it is said in F.A.N.G's Story Mode that M. Bison is immune to poison. But in gameplay, Bison can be poisoned by F.A.N.G just like every other opponent out there. However, upon winning, F.A.N.G accuses this Bison he just defeated of being an impostor. Thus, F.A.N.G knows it's an impostor because this "Bison" he just fought is not immune!
Several people complain that Akuma's Shun Goku Satsu, despite being hailed as the most lethal Ansatsuken technique, doesn't really kill anyone even when used in cutscene. However, one of the common teories is that this attack channels 16 Hells of Buddhism through the victim's soul - but when Akuma uses it in game, it deals 15 hits. It's quite possible he holds back its true power just like he holds his Shin Akuma form.
Juri's color motifs when she uses her specials change from light purple in SSFIV to black and white in V. In the interlude between IV and V, she loses her original Feng Shui Engine and has to work with a new one. The new engine probably comes with a different color palette.
Balrog's very status as a DLC character in V is this. What does he love more than anything else? His fight money. How do you get him as a playable character? By giving him enough money to convince him to fight for you.
With the exception of his Critical Art, Ed's special moves forgo the traditional fight stick motions that other characters have in favor of simple inputs. Despite him being a full-grown adult due to Rapid Aging, Ed is still Younger Than He Looks and probably still has a ways to develop as a fighter before he can perform special moves like the rest of the World Warriors, who are all either adults or have had more combat experience.
Why do Ryu, Ken, Sakura and Karin wear MMA Fingerless Gloves? Ryu fights constantly and would have to wear them in certain circuits, and likely got in the habit, Sakura imitates Ryu because she's her fan, and Ken and Karin don't want any special advantage or disadvantage on their rivals.
How did Akuma beat Ryu despite Ryu supposedly having gone stronger than Gouken and therefore, Akuma, as of A Shadow Falls? Well, before the events of V, he and Gouken fought with the winner claiming the cub. Due to Ryu being in Gouken's side as of V, its very obvious that Akuma lost the battle. How can Akuma compensate with this humiliating scratch to his win-loss record? By getting strongeroffscreen, of course!
M. Bison's elite assassination squad the Dolls is full of this. When you take into consideration how the Dolls came to be and how fond of women Bison is, you get the feeling that M. Bison had been doing alot more than just brainwashing them...
The nature of the Illuminati's experiments can be horrifying, with the implication about exactly who they use as test subjects. While Necro and Twelve's creations were bad enough, Chun-Li's storyline has the Big Bad (Wannabe) Urien kidnap a young girl for the sole purpose of experimenting on her (though thankfully it didn't happen). And then there's Urien's ending (if you play as him), where it's revealed that the Illuminati has a stockpile of hundreds of chemically-preserved brains dated from thousands of years ago, with some of them possibly belonging to children...