Fanon: It's usually accepted that the reason JB managed to deflect Satan's lightning bolt in the end with his guitar was because Tenacious D had actually managed to win the rock-off.
Fridge Brilliance: JB agrees to put KG up as a bet for their rock-off with the devil...despite the Devil having said that he literally CANNOT decline a rock-off challenge. However in order to send Beelzeboss back to hell from whence he came. Jables HAD to strike a deal with him of some sort. There would've been nothing to gain from the challenge on either side, had Jables not offered Kyle's soul to Beelzeboss in the first place. Beelzeboss feed on the souls of the tormented. It literally was "the only way". If the D had won WITHOUT doing this, Beelzeboss would be loose on Earth and would have taken control of the world.
Seeing as Beelzeboss threatened that they would suck his cock for awakening him on to the mortal plane, JB actually AVOIDED sucking cocks in hell, by specifically offering Kage's soul in case they lost the challenge, where they'd be both literally fucked. And seeing as they won they don't NEED to go to the big rock-off anymore because they just saved the world with their rock, and Beelzeboss pays their rent now.
Unfortunately for them, they didn't actually win the rock-off, they just reflected Satan's attack with JB's guitar (a symbol of friendship between JB and KG. Yeah, symbolic stuff tends to do well against the supernatural). In a deleted scene, they mooch off of Lee after the events of the movie. Or maybe the guitar only reflected the attack because they had won; it's not really clear.
Symbolic indeed. The guitar's neck is covered in symbols and iconography from practically every major system of faith.
The song at the end of Beelzeboss is the song Tribute is a tribute to, think about it.
OK, OK. Jables is clearly the greatest Chessmaster the world has ever known. He gave the Devil his terms: either he could have taken his sorry ass back to Hell and pay their rent or he could take Kage back to Hell to be his little bitch. In accepting this arrangement, the Devil agreed to shell out about $300 or take home a brand spanking new, rotund personal sex slave and go back to Hell either way.
It makes perfect sense that Beelzeboss will live in "The city of fallen angels", as Dio put it
Lawful Evil: Satan concedes to the commands of the Demon's Code that he has to agree to the rock-off challenge whether he wants to or not. He doesn't even cheat when he rocks-off either. He's that good.
Signature Scene: The Rock-off against The Devil that is the climax of the film is by far the most remembered moment of the film, in no small part due the "Belzeboss" musical number.
Strawman Has a Point: So, wait... rock and roll actually does come from the Devil?! JB's dad was right!
JB's dad was right about pretty much everything. He also said that rock would be a corrupting influence on JB, which JB blows off with "Rock is not the Devil's work! It's magical and rad!" By the time he's been living the rock lifestyle for years and learns that the Devil is in fact responsible for rock'n'roll, he thinks that's awesome and will do whatever it takes to snag that demonic power for himself.
Though if we want to get technical, the Pick of Destiny is actually a product of triumph over the Devil.
They Wasted a Perfectly Good Plot: There are about 15 minutes worth of Deleted Scenes which would have set up more jokes and allowed for better character development. Liam Lynch had them all cut to speed up the pace and allow for more gags, but there is a minority of fans who feel that it wasn't worth it, and both of those things would have benefited the movie by giving it a stronger emotional core.
The Woobie: KG has a couple short scenes in the movie designed to invoke this. One shows that he was bald as a kid and bullied for it, and another has a college kid throw a plastic cup at him after he bombs a performance. Bordering on Jerkass Woobie as he takes advantage of Jables for the first half of the movie, makes up stories to try to get with girls not half his age, and is a major slacker.