Hyperspace: Everything is a bright swirling mess, currents pushing ships this way and that, if you lose your navigation systems or your engines, you will drift off the plotted route and will, in all likelihood, be Lost Forever. Even in the event of full-scale war, very few of the races are willing to fight in Hyperspace for this reason.
To make it worse, there are even rumors that there is... something... living in Hyperspace. The rumors are true.
It Got Worse: once you find out that Hyperspace is not the only alternate dimension, and you begin to deal with things such as Thirdspace, which is home to... something much worse.
Dr. Franklin gives us a vivid first-hand story about how he got to watch a friend of his accidentally get spaced. Struggling, desperately flailing his arms and legs, trying to breathe... and death by spacing is not a swift experience.
Death by Mind Rape. And if in your dying moments you should think of your friends or loved ones, they will be put in danger as well.
The na'kaleen feeder from Grail combines this with the Soldier of Darkness. The Centauri quarantined an entire planet just to contain the threat this entity posed, and they're one of the more advanced races of the B5-verse.
The entire concept of the Minbari War: one man's tragic mistake nearly dooms his entire race to extinction.
The fanaticism a species would have to condone such an action is scary too.