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.
Arc Fatigue: The final arc is getting this for the repeated Say My Name between Kurono and Tae as well as the slow pacing.
Angst? What Angst?: Cloned Kurono seems to get over the fact that he is not the "real" one rather quickly. Of course, living with an Idol Singer who falls head over heels for him obviously helps.
Original Kurono is also quick to accept the fact that he has a clone and is not too shocked when faced with said clone. The two resolves their love problems within one page. Probably justified since the alien invasion is imminent, and both of them just want to live peacefully while they still can.
Big Lipped Alligator Moment: That entire scene in the Room of Truth. Some extremely powerful god-like aliens deliver soul-crushing Breaking Speeches to the heroes before sending them right back to the Giant Alien mothership to finish the series. Said god-like aliens are never seen or mentioned again, and even without the abrupt nature of the ending, they seem to exist solely to justify the Gantz technology, and pretty much explicitly say "So ... yeah ... you did what we wanted you to do, we're done with you now, your 21 grams of data can have a nice eternity being reincarnated, or not, we don't really care either way."
Designated Hero: Kurono is an asshole in the early parts of the story. He gets better.
Designated Villain: Some aliens only try to live through normal lives, without bothering anybody.
Does This Remind You of Anything?: The American Gantz team holds giant aliens in stress positions, sexually abuses them and forces them to abuse each other for their amusement in a way similar to Abu Ghraib and other War on Terror scandals.
Ensemble Darkhorse: Nishi. He came 1st in the official character popularity poll and has a ton of (often yaoi) fanart dedicated to him in the Japanese fandom. He's pretty popular in English-speaking fandom too. Some would say this has devolved into Draco in Leather Pants treatment. To others he's The Scrappy. You either love him or hate him.
Fan-Preferred Couple: Kurono/Reika...with the interesting twist of having two Kuronos now. Other fans see her as a scrappy.
Hollywood Homely: Tae. Her face is a bit unusual compared to most other characters, and it's not entirely clear at first whether this is meant to suggest that she's ugly, that she's not pure Japanese (some mainland Asian peoples have that kind of facial structure), or that she's mentally challenged (it's also similar to the characteristic appearance of people with Trisomy 21). That said, there are plenty of characters in the series that are actually ugly (most "thuggish" characters ... the Yakuza, the school bullies, the bikers ... tend to be hideous), and Tae isn't one of them.
Shion Izumi has done more than his fair share of dog kicking (just ask the 387 people he killed just to selfishly get himself back.) But all those 387 deaths were not as irredeemably heinous, nor tragic, as when he cold-heartedly killed Tae in Volume 16. And while it was Gantz who assigned the mission, it was clear that one could choose not to kill her, much less in such a sudden, vile way. Rest assured though, the vampires slay Izumi four volumes later and Tae gets revived by then.
The Truth: Now we shall prove that human beings are merely "things..."
Rescued from the Scrappy Heap: Nishi started off as an asshole who, while experienced, was constantly seen disappearing from battle and leaving much of his colleagues to die. That all changed in chapter 341 where he kicks the viral monsters' asses and saves Takeshi from the explosion at the department.
Squick: During the Osaka arc, some Osaka team members are seen raping various female demon/aliens, which is bad enough. But then you see one stuffing his hand in the mouth on the back of one's head. That took it to new levels of grossout. Not to mention how many times they have pointless ECUs of people's spit coming out of their mouths; not total grossout, but it'd be nice if there was less of it.
The female demon/aliens being raped is a Futakuchi-onna and a Rokurokubi.
They Wasted a Perfectly Good Plot: The Vampires sub-plot itself is also casually forgotten, when their involvement in the alien invasion side-by-side with the Gantz players could've been extremely interesting
Wangst: There's a reason the page image for the term is of Kato. He's actually strong, tolerably smart by the series' standards (which, granted, aren't all that high), and fairly effective whenever he manages to stop boo-hoo-hooing about ... well, pretty much everything ... and actually do something other than just whine about it. Which is to say, almost never, at least up through the Buddhist Temple arc.
The Woobie: Takeshi, for officially having the worst childhood ever. The heartbreaking part? He's still a toddler.
Writer Cop Out: This seems to be the common emotion among the fanbase toward the manga's finale.