* AlwaysChaoticEvil: Many of his villains. According to Ennis himself, this is a direct reaction to the concept of "JokerImmunity" that pervades the comic-book medium. He once phrased it in a ''Wizard'' interview as writing villains who are "worth the bullet"; he doesn't enjoy works in which the hero proves his moral superiority by letting the villain live, as that's usually a choice on the hero's part that will get someone else killed at some point down the road.
* CrossesTheLineTwice: Ennis has an extraordinarily dark sense of humor and a penchant for sex jokes, which turns many would-be readers off from his work. While he's not unusual in that regard among other UK-based writers (Creator/WarrenEllis in particular can give him a run for his money), Ennis is the one who's built a reputation off of it.
* PromotedFanboy: He [[http://www.bleedingcool.com/2009/08/24/garth-ennis-when-2000ad-was-the-future/ was a big fan]] of ''ComicBook/TwoThousandAD'' and especially ''Judge Dredd'' as a kid.
** Played with on his run on ''ComicStrip/DanDare''; in an essay to the collected edition, he openly acknowledges that while he respects the character he has no particular sentimental attachment to him; he does, however, appreciate the values that Dare's creator imbued him with, which attracted him to the project.
* WriteWhoYouKnow: A lot of his best-written characters are Irish.