A lot of people died in Dredd 3D. It's worth mentioning that up top. This was a very violent film which featured a number of truly horrific and creative things being done to the human body.
And it's something that will be discussed for a while--the gruesome and graphic nature of the film being what it is, and the box office on pace to be pretty disappointing in spite of great reviews, there will be some discussion about whether a PG-13 rating could have saved it from financial disappointment. The answer's no, by the way, because it's unlikely that this particular movie could have been made under the limits of a PG-13, and it's hard to imagine any iteration of a Judge Dredd story being told in that forum and being a success.
HUGE spoilers ahead, folks, so don't read this until you've seen the movie. And really--why haven't you? It's brilliant.
What were some of the more memorable and gruesome moments that will stick with viewers for a while?
The motorist Dredd kills
Following a high-speed chase, Judge Dredd takes out the last surviving member of the car he was pursuing. It was a cool sequence, but mostly forgettable, excepting one thing: When the scene moves directly from Dredd's judgment to the Hall of Justice where he's being asked about the events, you get a cold chill for a moment, thinking it's going to be one of those movies where he's dogged by a superior officer who "just doesn't get it" the whole time.
Luckily, the movie sidesteps that right away and Dredd was being summoned so they could saddle him with a trainee instead.
The murder victims
The murder victims that kick off the film are kind of an easy one, since it's the event that precipitates the whole plot, but they're memorable for way more than that. Dosed on a drug that makes them perceive time in extreme slow motion, they're then tortured and killed (at regular speed), which means they're essentially experiencing that agony for hours or days before they finally fall to their deaths from a height of 200 stories. That's messed up, even for a bad guy to do to people.
There's a Slow-Mo shoot-out that was used heavily in the promos, and which an animator had inadvertently leaked the animatics for at one point, showing some of the more brutal stuff. There's no one person who dies in a spectacularly memorable way in that sequence, but the whole thing just sticks out as this brilliant flurry of violence. We'll give honorable mention, I suppose, to the character whose cheek is penetrated by a bullet in slow motion, since that's just got to be crazy painful--and he didn't escape the room, so we can call the death.
One of Ma-Ma's most memorable agents in the film, Kay's death is memorable first because it was so stylish and well-done compared to a similar scene in 1995's Judge Dredd (the Lawgiver "backfires" and secondly because throughout the film, he cowers from Dredd but tries on several occasions to stand up to Judge Anderson. Whether it's because she's a rookie, a woman or a mutant is unclear, but in any event there's a conflict that goes back and forth between them throughout the film. It's battles within the larger war, so to speak, and she wins each of them, usually making him look foolish in the process. Finally, it looks like he may finally have her right where he wants her--but apparently the awareness that EVERY Judge Dredd fan has about how the Lawgiver works is not common knowledge in-universe--or else messing with a psychic for a few hours has just scrambled his brain.
In any case, Anderson keeps her perfect record of winning every conflict she has with Kay--but he's not around for her to gloat about it.
While Ma-Ma's death is little more than a callback to the murders she committed at the beginning of the film, it takes the same idea (minus the flaying, since Dredd isn't The Spectre and isn't really in the business of that level of symmetry) and turns it up to eleven. Where the initial victims came down on the story like a ton of bricks, we see Ma-Ma go down, experiencing it in glorious slow motion just as she would since she's been dosed with Slo-Mo prior to the push--and the final shot, as we see her landing on the camera, is just flabbergasting. It's disgusting and brilliant all at once, and was the single most memorable shot in a very well-photographed film.