Since I've been in a Star Trek mood lately, let's go over some of the lovely ways you can die in space.
As I mentioned before, simply being a redshirt guarantees you a horrible death, typically at the hands of the Monster of the Week.
In Star Trek: The Motion Picture, a nameless science officer falls victim to a transporter accident. This line pretty much sums it up: "Enterprise, what we got back didn't live long. Fortunately..."
In Star Trek 2: Wrath of Khan: I shudder to think of the horrible torture Khan subjected to the scientists on that research station before killing them. Apparently the film's novelization goes into much greater detail but I haven't read it. Also, Spock dies of radiation poisoning. That's a pretty lousy awful way to go.
In Star Trek 3: Search for Spock, we get treated to the lovely sight of a klingon grunt getting disintegrated from the inside out all while screaming in agony.
In Star Trek 6: The Undiscovered Country. Getting shot with a phaser while floating helplessly with no gravity would be a lousy way to die.
Star Trek: First Contact: This is the first time, at least as far as I know, where we actually get to see the process of being assimilated by the Borg. At one point a crewmen is shown writhing in obvious agony while hi skin changes color after being injected with Borg nanoprobes.
How about the admiral in Star Trek; Insurrection who gets subjected to the face-stretchy machine thingy. Ouch.
In the 2009 Star Trek film, an early scene shows a crew member getting sucked out through a hull breach into the vacuum of space. The camera lingers just long enough to show their flailing body bounce off a phaser bank.
Also, the entire population of Vulcan gets sucked into a black hole and presumably reduced to subatomic particles.
In Star Trek: Into Darkness... (spoiler alert!)
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Ships in Star Trek use inertial dampeners to protect the crew from being pulverized by the sudden acceleration/deceleration of the ship while it's traveling at warp speed. Keeping that in mind, there is a scene where the Enterprise is attacked while traveling at warp speed, during which a massive hole is torn in the hull resulting in several crew members getting sucked out into space. While the ship is traveling at warp... You go from a standstill to suddenly being exposed to the vacuum of space where you are no longer protected by the ship's inertial dampeners. Basically, your body would be instantly eviscerated by the massive trauma.
Also, there's this lovely gem...
Khan (while squeezing Admiral Marcus's head with his bare hands): YOU SHOULD HAVE LET ME SLEEP! (insert sickening crunching sound here)
In happens offscreen so we thankfully don't see it. But still, that awful crunching sound, accompanied by the horrified reactions of the characters drives the point home. Sweet dreams!