These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: High School D×D
Anti-Climax Boss: Even being turned into a chimera, Kiba one shots Freed like nothing.
Base Breaker: If there's one thing fans really can't agree on, it's the Gremory-Sitri Rating Game. Was it thoroughly justified that the Gremory team had a limiter working against them, thus giving them a real wake up call that just powering through everything isn't really all there is to it, or was it unfair to the Gremory team that they had a limiter on them when they couldn't show off what they could do now after their training? Or you're probably on the third faction and thought that the limiter made the Sitri team look weak, never mind Souna being able to adapt things on the flybecause only the protagonist has the right to be aCombat Pragmatistright? Part of the problem really is because Issei, the protagonist himself, gets taken out halfway through the game because of said limiter imposed when most series would have the protagonist be present all the way to the end.
Crazy Awesome: Seriously, if he wasn't the protagonist, Issei would be dead by now with all the crazy stuff he can pull off. To wit, he has: done a rescue mission to Asia by punching out a Fallen Angel so hard she bursts through the wall (out the window in the anime), barged in an engagement party full of devils to take back Rias, invented Dress Break and Bilingual out of pure lust, defeats Riser, an immortal phoenix, not with the use of his newly-found dragon powers, but by using a holy cross and holy water which he can't even use because he's a devil, but is capable to do so because his arm is a dragon's, powers up with lust and by using Rias' breasts, accidentally summons the Breast God (well Breast Fairy but still definitely counts), is capable of breathing fire to a shadow-using opponent, and was able to make the past users of Boosted Gear come back to the light and make them stop relying on Juggernaut Drive with breasts. If there's one thing Rias recruited to her group, it's the fact that she recruited the craziest character of the series. No one out-crazies Issei at all in this series. And yet somehow, he's also the Only Sane Man in the group (when he's not doing his lecherous ways).
Rias may count to a lesser degree too. Who's crazy enough to be willing to bare her breasts open just to let Issei get a power up in public viewing!? Also renovate Issei's house from a simple house into a six-story mansion with a three-story basement and an elevator overnight!?
Azazel too can count as one. The craziest idea he pulled off is that to capture the Issei clones he accidentally unleashed, he suggests the idea of using fishing on them. With porn as the bait.
Forget Azazel himself, the entire Grigori institute counts.
Die for Our Ship: Despite being a harem and everything, people seem to not like Asia as much as the rest of the characters because according to the fans, she's too clingy to Issei to share with everybody else.
Ophis is practically well-loved by the audience too.
"Funny Aneurysm" Moment : An in-universe example. One of the short stories of the series was about the Issei clones that Azazel had played a prank on Issei. It was mostly funny until the part where they had to use the memory-erasing machine Azazel had invented (at best, they could only erase the memory of the clones running around but didn't erase the fact that an Issei clone did it, leaving the victims thinking the real Issei had done it to them because it would damage their brain memory). At that point in time, it was hilarious, albeit a bit unfair to Issei who didn't really deserve all that. Fast forward to volume fourteen, and that memory-erasing machine incident suddenly doesn't become funny anymore thanks to the student council group using it to wipe off the memories of the students were the victim of a rouge wizard terrorist attack at the school, which leaves students traumatized of something but not what. This left a bitter taste to the student council and the protagonists.
Hilarious in Hindsight: Vali notes back in the fourth volume that the current red-white dragon rivalry was a bit unfair because Vali's the descendant of the old Lucifer and Issei's the son of a common Muggle. Vali wishes that Issei's parents were a bit special so they can have a more interesting match. Now that Great Red and Ophis are both Issei's parents by proxy by creating a body for him from Great Red's flesh and Ophis' powers, let's just say Vali's wish came true. Though seeing as Vali's a Blood Knight, he's probably pleased with the development.
The volume one afterword has the supposedly final battle will be between a guy who can breath flames and Issei supposedly shooting out a blaster. Now read how the final clash between Issei and Jabberwocky went in volume 12.
Memetic Mutation: Somewhat a meta example: if any volume is delayed for its release, expect people to post about Ishibumi and trains. Fandom just went with it.note It's because Ishibumi keeps himself hitting his shoulder on the train doors.
Memetic Sex God: For the male characters, it's Azazel and with just one pic from the anime, Lord Gremory.
Tear Jerker: Issei's death in volume 11 was definitely one.
Tough Act to Follow: Not the show itself, but the show-stopping pole-dance ending-song sequence Study X Study from Season 1, with its energetic and upbeat song and astoundingly fluid and well-designed animation (all animated by one man, mind you) set such a high standard (and became a Cult-Favourite amongst fans) that the second/third ending sequence from Season 2 simply couldnt top it, and hence the crew didn't even try.
What an Idiot: Most people seeing Issei being his usual self. On another note, the Two Heavenly Dragons (Ddraig and Albion) decided during the great war to fight God, Satan and the leader of the Fallen Angels.... they were quickly dismembered and the Sacred Gears are what's left of them stored.