Many people on Less Wrong believe reducing existential risk is one of the most important causes. Most arguments to this effect point out the horrible consequences: everyone now living would die (or face something even worse). The situation becomes even worse if we also consider future generations. Such an argument, as spelt out in Nick Bostrom's latest paper on the topic, for instance, should strike many consequentialists as persuading. But of course, not everyone's a consequentialist, and on other approaches it's far from obvious that existential risk should come out on top. Might it be worth to spend some more time investigating arguments for existential risk reduction that don't presuppose consequentialism? Of course, "non-consequentialism" is a very diverse category, and I'd be surprised if there were a single argument that covered all its members.
Most non-consequentialists are not indifferent to consequences. For example, they might believe in punishing drunk drivers irrespective of whether they run into someone - but if they drive drunk and then actually kill someone, that is still highly relevant information.
Egoists are more of a problem from the perspective of this cause, I believe.
That's an inferential step further, but those could be swayed by the prospect of a very very long life. It's a reaaaly long shot, but existential risks are a barrier to personal immortality.