Doom 2099: Doctor Doom takes over the united states, gets injected with alien drug blood, goes crazy, gets involved in a nanotech deus-ex-machina arms race with a proto Spider Jerusalem Corrupt Corporate Executive and his insane cannibal Captain America clone, becomes a plasma-shotgun toting cyberpunk revolutionary and saves the future from itself.
Gravel, including the Strange Killings mini-series and the eponymous ongoing. The book has since been rebooted into a new volume, Combat Magician, with a different writer.
Scars: One of his earliest works for Avatar. A police officer finds himself dealing with a serial killer when that killer's latest murder strikes very close to home. Ellis has admitted the book came directly out of his anxiety as a new parent, making it the virtual embodiment of the Adult Fear trope.
Ultimate Comics Armor Wars: Tony Stark tries to find his stolen tech, while Warren Ellis is having fun.
Secret Avengers: A six-issue run (so far; he's stated that he'd be willing to write more). If you thought Astonishing X-Men was Crazy Awesome, you have yet to meet the gleefully insane Moon Knight and Pilot Marko. Not to mention his Beast.
Hank McCoy: I can't fire a gun! I have paws!
Tropes associated with Warren Ellis and his works:
Artistic License - Biology: After he explained the difference between normal and artificial mutants (or were they mutants from alternate reality? Probably both) in his first Astonishing X-Men story, people at Scans Daily pointed out that genetics don't work that way. Ellis admitted his mistake.
When Ellis wrote Iron Man: Extremis, he explained the eponymous magic bullet (a single injection which would turn ordinary mortals into supermen) as a "Data package contained in a few million carbon nanotubes, injected directly into the brain". The information package would then rewrite the repair center in the brain — that is, the part of the brain which keeps a complete 'map' of our organs and functions. "The brain is telling the body is wrong"... and it compliantly changes according to the Extremis instructions. Perhaps needless to say, there is no "repair center" (although the "sensory homunculus" seems a little bit like what is described). Later writers retconned Extremis into a viral package, which is at least borderline believable.
And in Supergod it is a point that "mushrooms only grow on dead things". Which, well, they do not.
Atheism: Ellis does not believe in God or gods, nor do most of his characters. Especially his Author Avatars like Spider Jerusalem.
Attention Deficit Creator Disorder: It varies, but this often turns out to be a good thing; some of his best work is stuff he came up with and wrote while he was supposed to be writing something else. Avatar Press probably has a red phone just for him: "What's that, Mr. Ellis? You want to have SuperIron Man kill the POTUS? Shoot it on over and we'll print a run, see how the fans like it. We'll keep the presses warm, too, because they probably will."
Author Appeal: New Media. His run of JLA Classified is basically an Expy of Global Frequency - the Leaguers simultaneously find something so weird their brains go blue-screen, so they all call up Martian Manhunter telepathically. He then calls up Oracle of the Bat-clan, who powers up all her computers and digs in.
Wonder Woman: J'onn, this is Wonder Woman. I need a consultation.
Green Lantern: J'onn, this is Green Lantern. I could use some extra brains here.
Martian Manhunter: I Hear You All. This is J'onn J'onzz on the Lunar Watchtower, activating the Justice League Telepathic Link.
Oracle: This is Oracle in the Gotham Watchtower. Information mining system on. You Are On The Global Frequency. Justice League Is Go.
Secret Avengers also bore a lot of similarities to Global Frequency, complete with "mini-teams" similar to the GF teams.
He's also very interested in space-flight, trans-humanism and the Singularity, all of which tend to pop up in his works to some degree.
Nanomachines and corrupt mega-corporations pop up all over the place in his work.
Many of Ellis's protagonists are partially or wholly defined by their addictions: smoking, drinking, coffee, etc. To listen to him talk on his various blogs and newsfeeds, Ellis himself isn't far behind them; he's unsuccessfully attempted to quit smoking several times, drinks enough caffeine to kill a buffalo, and really likes his whisky. It is arguably not an accident that many of his best-known original characters are also chain-smokers.
With Pete, it actually gets fairly weird, considering Wisdom goes on to take Kitty Pryde's virginity, when Kitty was, at the time, only just turning 18 (and even there, it was Ellis who established her as such, as her actual age wasn't clear before and previously she'd been depicted as 13-15). It leads to some really weird implications considering he fully admits Wisdom being an Author Avatar.
Author Filibuster: Related to author avatar above, as the said characters are always ready to explain at length their views, sometimes at odd moments.
Berserk Button: Holy shit, does he hate dogs. Ellis will rant about this on his website or Twitter if given an excuse, but he finds a lot of reasons in his work for dogs to die in ways that he finds funny: accidentally crushed underneath a falling unconscious goon, "culled" for being feral and sentient, infected with a zombie gas, etc.
Beware the Superman: He has to like this trope very much - Black Summer, No Hero, Supergod are all about it, echoes of it can be found in Planetary, Authority and Thunderbolts, and one of his first Stormwatch stories quoted it explicitly.
Doing It for the Art: Pretty much his entire work on Avatar Press. While their sales are extremely miniscule in comparison to even the most unpopular mainstream comic, Ellis manages to publish some of his most distinctive work for Avatar.
Eagleland: Any of Warren's works set in America will feature people who are either selfless, heroic martyrs, or abusive, misanthropic jackasses; and locations ranging from a splendid New York City to the most rundown trailer park imaginable. Ellis has said in Q & A's that his feelings on America are mostly that while he thinks it is the greatest country in the world, he also feels it has the potential to be far greater, and is annoyed by this.
Somewhat inverted in his Secret Avengers run: Americans ends up being the only people moral enough to step in and stand up for some African villagers being hunted and abducted, and in the end of the issue it turns out the ones responsible for the abductions were all British, with Steve suspecting they may have government ties.
Executive Meddling: Averted when he wrote GI Joe: Resolute. According to him, the Hasbro executives were generally easy to work with and while they didn't allow Cobra Commander to wipe out Beijing as his original plan, they let him wipe out Moscow.
Expy: Eli Warren, created by Kieron Gillen in his Iron Man run, shares a name, an accent, an appearance and an obsession with Transhumanism with Ellis. (Of course, given that he lives in the Marvel Universe, he's more inclined to act on that obsession...)
Fandumb: The consistent belief that Ellis 'hates' superheroes. Often due to people looking at something like The Authority (when Ellis' run was much, much less severe than say, Mark Millar's) and getting the wrong impression entirely. Either that, or they confuse him with Garth Ennis.
A column from Ellis shows that it's not that he hates superheroes, but rather that he never grew up with them the same way other comic fans did. Thus, his view on characters like Batman or Superman would mirror how a man would suddenly discover manga or pulp comics.
See this very page for many repetitions of the idea that Planetary #8's John ConstantineExpy was transformed into Spider Jerusalem by Ellis. Word of God is that Ellis' intent was to have the character turn from John Constantine into King Mob. It was Cassaday who decided to keep going and make him Spider. (Not that Ellis didn't enjoy the idea, but it wasn't his.)
They're also hilariously ineffective, as seen in The Authority and Planetary; In the former, no government or political power can stand against the threats he lines up, and in the latter, the world's progress is actually controlled and allowed to progress at a snail's pace by an evil pastiche of a popular superhero group.
Also in Global Frequency, where The Government has usually created the threat of the week but fucked it up to such a degree that they're unable to handle it when it gets out of control, and thus have to rely on the global civilian network to clean up their messes.
Knight in Sour Armor: His favored protagonists tend to be world-weary and cynical chain-smokers living in Up to ElevenCrapsack Worlds who nevertheless possess rigid moral cores and a commitment to doing the right thing despite themselves.
Name's the Same: Ellis shares the same name with an Australian music artist who frequently works with Nick Cave. It's gotten to the point that several times a year he posts and Twitters a message that usually goes along the lines of "Once again: I am NOT the Warren Ellis who plays with Nick Cave. I am a comic book writer."
Ellis claims that he occasionally gets phone calls from a drunken Blixa Bargeld (a bandmate of the other Ellis), making this same mistake.
No Ending: A fair few of his shorter works don't end so much as they stop.
Schedule Slip: When reading his ongoing work, prepare to wait months, if not years, for the next issue to come.
According to Ellis via his message board, his main writing computer and backups failed back in late 2007 (the computer going out in a puff of smoke soon after the backups died), and the data-recovery company that he sent his hard drives to promptly vanished off the face of the Earth after the owner died while on an operating table in Europe and the employees scattered to the four winds, leaving behind only a boarded up empty space where the offices used to be. Many of Ellis's current projects were on those hard drives, including Fell and newuniversal. He has since then set up a FAQ thread on the Whitechapel forums where he explains the fate and (possible) future of many of his projects that were directly affected by the crash.
Shown Their Work: Whether he's writing about bleeding-edge speculation about physics, biology, and technology; world history; or the characters of an established Shared Universe, Ellis demonstrates a knowledge of even obscure or minor details.
Sliding Scale of Idealism Versus Cynicism: Generally pretty cynical though to what extent varies from story to story. There's often a small glimmer of hope at very least, regardless of how dark the plot is. His Avatar Press Superhero trilogy is notable for veering hard right into the furthest reaches of the cynical side of the scale, all ending in a complete Downer Endings.
His Switchblade Honey is this to Star Trek - it shows a future where the exploration of space is handled by a bunch of insane egomaniacs, which leads to a war with a much more powerful enemy, which humanity is losing. Heroic idealists, who would become great heroes of Starfleet in Star Trek, here end up in prison for opposing the corrupted system.
Status Quo Is God: Criticized a lot, especially in Planetary, which really strongly criticizes Reed Richards Is Useless and similar tropes used by editorial mandate to make their worlds closer to ours and preventing any changes. Even John Constantine has been criticized for not really changing since his first appearance (oddly Ellis suggested Constantine should become more like Spider Jerusalem).
Thou Shalt Not Kill: Averted - most of his heroes don't have those kind of issues. There's an oddly frequent idea in his work, that some people contribute nothing to the world but sorrow and thus need killing. It's most clearly articulated in the Hong Kong issue of Planetary.
He occasionally causes controversy when he does work for hire, because Ellis flatly refuses to write a character with an iron-clad code against killing. The closest he comes is Swift in The Authority, who under his pen will kill if necessary but doesn't want to. (This was not a trait that later writers adopted.) There was a mild controversy after Ellis's first issue of Secret Avengers, after he had Hank McCoy make an impossible choice: either use an improvised dirty nuke to kill a few hundred members of the Shadow Council, or lose the city of Cincinnati to a weaponized time machine. Hank picked the first one, but not without comment or complaint.
As a matter of fact, it transpires that this was, if not impossible, than at least hard to swallow: in order to make Stark more badass, Ellis first made him a bitch. Extremis in a nutshell - a Right Wing Militia Fanatic shoots up with Nanomachines, and the resulting Hulk-level sociopath Curb Stomps him. The hero who once flew a nuclear reactor the size of a city block in the process of meltdown out to sea ends up in severe danger of being killed by a redneck using a car as an improvised sledgehammer. Stark survives to shoot up on nanomachines himself, enabling him to plug his brain into a brand-new suit, then goes back in swinging an earthmover bucket like a foam rubber bat.
Writing for the Trade: Averted as all hell. While noting that he probably has more trades out than any other writer (it's not uncommon for comic book shops to have an entire shelf of his work), Warren has strived to make his comics such as Fell, Planetary, or Global Frequency as self-contained within each issue as possible.
You Keep Using That Word: In at least two instances (JLA Classified: New Maps of Hell and Planetary), Ellis has used "ancestors"(those you're descended from) when the text clearly should have used "descendants"(those descended from you).