Anti-Climax Boss: In SB3, before the expansion, Muneshige is hard to beat even while weakened, but the Xavi tank can be destroyed in secondsnote otherwise, it's pretty much inoffensive if you stay directly in front of it, provided it isn't singing while you do that and Sorin himself dies after one hit.
Badass Decay: Thanks to Character Exaggeration. Imagawa Yoshimoto is much more doomed than his Koei counterpart, who at least has stayed throughout that series and is speculated to be Obfuscating Stupidity. Here, he's a 100% buffoon and cowardly, and because Nobunaga is a 100% SOB, he's more likely to die ignobly, and because this series run on Popularity Power, he's all but completely vanished since the second game expansion. That's some way to go for a competent daimyo before Okehazama.
Base-Breaking Character: Mori Motonari. On one hand, he's definitely one of the more popular characters in the game and has quite the personality and capabilities to back that up. On the other hand, it's his personality that made him quite hated, as being a selfish jackass commander, successful or not, is still going to leave a bad taste on people's mouth and even more, he probably is one of the people that strayed the most compared to his historical counterpart: A famous personal-event by historical Motonari was that he gathered his sons, told them to break off one arrow, they succeeded, but then he told them to break three arrows at once and they failed, intending to teach them a lesson of unity: One person is easier to break than a group of people banding together. Basara's edition of Motonari probably would prefer to DIE before giving such anecdote to anyone, as he's basically convinced that he's the only thing that matters in the world because of his genius that he'll have zero time for family and friendship.
Broken Base: In the eve of Yukimura-den, Word of God has it that there won't be a new female character... and the base spectacularly broke, with one camp saying that at least the extremely fun over the top gameplay factor is still in tact and it makes sense that female emancipation did not really exist in the Sengoku Period, and another camp crying out how Koba is just pandering to the yaoi fanbase and gives a complete middle finger for those who appreciates female character, and considering to jump ship to Koei, with claims that at least they're fairer to the opposite sex.
Motochika and Motonari were popular NPCs that got promoted into playable characters in the second game. From the second game to the expansion pack, Kojuro, Nagamasa and Oichi. But mainly Kojuro (the one who spawns the most fanart between those three). And there was much boo-ing when he got Demoted To NPC.
And special mention to Oichi. Mind you, this is a series dominated with pretty and manly boys, but the Oichi fanbase is very very solid when compared to other female characters, which includes her being the only playable female in the fighting game and returning in 3.
Mitsunari, while actually being a main character in 3, proves to be so popular that he took over Yukimura as the poster boy as of Sengoku Basara 4.
According to popularity polls, the most popular newcomer in 4 is Matabe, despite the comparatively little focus he got in-game. In fact, one popularity poll had him rank 5th out of every playable SB4 character, behind only Mitsunari, Masamune, Ieyasu, and Yukimura.
Mitsuhide is fond of Nohime, the only person he shows any kind of sympathy when he kills them, on top of repeatedly calling her "beautiful" and using First-Name Basis. She just finds him creepy.
Let's not forget how he complimented Motonari's looks in SB2... only to add that he wanted to cut such a handsome figure into pieces.
Nohime herself has a habit of making suggestive remarks to younger men like Yukimura, Keiji and Musashi.
Disturbingly enough, Oichi has this with her brother in the third game, especially how she strokes his face as they return to hell together.
Yoshitsugu displays an unhealthy interest in Oichi's dark powers, and finds her positively charming. Kanbe points out he has horrible taste in women, to which he agrees.
He sometimes appears to be making advances at Motonari, though this may just be deliberately trying to annoy him.
Motochika with his rival, Motonari. Although in the games they are sworn rivals, the official artbook for the second game sees them leisurely drinking sake together.
Hisahide certainly seems to relish the idea of beating up Kojuro in Heroes. There's also the way he whispers salaciously to Mitsunari when he's got him pinned underfoot in Utage.
Yukimura's and Masamune's feelings toward each other are... complicated, to say the least.
Game-Breaking Bug: Sengoku Basara X, the fighting game spin-off, probably has too many to list. The result is a game where almost every character has hilariously infinite combos that result in both characters flying off the top of the screen.
Ho Yay: Unlike the Dynasty Warriors series which has a good share of both yaoi and straight doujins, SB is practically flooded with yaoi (with Masamune/Yukimura, Kojuro/Masamune, and Motochika/Motonari being the most popular) with the occasional Oichi/Kasuga hentai. Makes sense as Sengoku Basara doesn't have many female characters.
Macekre: The first game was called Devil Kings in the U.S, the producer attempting (and failing) to make a Darker and Edgier version.
Narm: One example of a result of the Macekre on Devil Kings is one of Nobunaga/Devil King's quote directed to the enemy: "Bow down before me, you idiots!"... which made him sound more like a frustrated boss Surrounded by Idiots than a menacing devil king about to assert his authority to lowly beings beneath him.
Nightmare Retardant: Oichi's static in the third game makes her sound more like a broken radio than anything actually scary.
That One Level: Sawayama Castle in 4 where you have to take 5 siege rams in total (3 at the first section and 2 at the second) to their destinations and each ram moves so atrociously slow and can't take many hits, forcing you to babysit them. Fortunately Sumeragi gives them a huge speed and health recovery boost, making the stage a lot less tedious.
That One Sidequest: From the third game, defeating and collecting all the fugitives. You have to equip a special bodyguard in order to find them, they have a habit of popping up out of nowhere and disappearing just as quickly, and hit surprisingly hard with a strong defense.
They Changed It, Now It Sucks!: Shortly before SB3 came out, many fans were baying for Kobayashi's blood for overloading the game with NPCs. Some still are.
Things improved with Sengoku Basara 3: Utage where many characters (including Kojuro, Tenkai and Matsunaga of all people) returned as playable characters.
Akechi Mitsuhide is depicted as a particularly depraved madman when he goads Nobunaga Oda's brother-in-law, Azai Nagamasa, into fighting the hotblooded hero Date Masamune. Mitsuhide diverts a squad of riflemen to fire on his own allies, having also brought Nagamasa's wife Oichi along. He ostensibly allows her to try to warn her husband before having him shot before her and relishing her pain. Mitsuhide is a vicious killer as well, serving Nobunaga only because it allows him to satisfy his bloodlust on the innocent and any warriors he finds. After the first season, he seeks to resurrect the Demon King, wanting nothing more than to watch the world burn.
And manly bonding sessions consisting of screaming names while punching each other in the face.
One-Scene Wonder: Kennyo, Yoshihiro, Xavi, and Itsuki in Season 1. The first three while they're slain by Nobunaga's men. Yoshihiro is back on his feet in Season 2.
Seasonal Rot: Judge End. The series is receiving a lot of criticism for its inferior animation when compared with Sengoku Basara and Sengoku Basara 2 by Production I.G. In addition to cutting the action back to a fraction of its former glory, introducing several annoying and useless characters and replacing the rocking soundtrack with vapid ear torture. The DVDs also got poor sales in comparison.
The same could be said for how Kasuga, Matsu and Oichi were portrayed as incapable in this show, in comparison to the video games.
A small part of the fandom was pretty confused by Yukimura's new hair style in the revealed character art for Judge End/Judgement.
As mentioned above in the Seasonal Rot entry, Judge End's animation is pretty much universally hated due to it's drop in quality compared to the work done by Production I.G. for Season 1, Season 2, and The Last Party.