Before I explain my opinion, first in a few sentences what the novel is about: Meursault is a young Frenchman living and working in colonial Algeria. He tells his own story, in a sort of minimalist, Hemingway-type "macho" tone. In the first part of the story we follow him as he goes to the funeral of his mother, visits the beach, and meets his girlfriend as well as other people who live in his apartment building. We notice that he does not care for others, nor for his job, and is not interested in anything whatsoever. Finally, he happens to murder an Arab young man, again for no obvious reason (a group of Arab youths has threatened his "pal," a neighbor from his apartment building, who apparently has acted as a "pimp" for the sister of one of them. But there was no reason for Meursault to become involved, as he was anyway indifferent towards this so-called "pal" as well). The second part is about Meursault's trial and subsequent death sentence. He is given the maximum punishment because he shows no regret about his deed (which he is unable and unwilling to explain) and his general behavior before the crime is interpreted as being devoid of normal human feelings. Meursault is not so much an outsider as someone who is truly a "stranger" to his own people.
Idea (1) is outdated because we know at least since the middle of the 19th century that there are no gods, that the universe has no higher purpose and that our existence (the evolution of conscious life) is just coincidence. Except for people born in an orthodox religious environment, there is nothing new about this statement anymore. It is a generally accepted fact of science and at least Europe has moved on.
Idea (2) is outdated because social conventions have changed and today people are free to be as egotistic and asocial as they want - as long as they don't hurt others. In case of a criminal trial, this attitude will in most parts of the world not lead to a more severe punishment.
Idea (3) is outdated because the death penalty has been abolished in the whole of the EU, making the "old continent" one of the most civilized parts of the world.
And I might add a fourth point where Camus is now outdated, this time through an omission: the fact that the murder central to the book happened in a colonial situation, where the killer was a French colonialist and the victim an Arab youth, is not addressed at all, something which today would be unthinkable.
Of course, the novel is so outdated because it is a novel of ideas. As soon as the ideas have become stale, the book degenerates into a museum piece. I therefore fully share Nabokov's often voiced objections against this type of literature!
But my greatest objection against the book is the character of Meursault - and this is an artistic point. Apparently, Camus wanted to depict an individual free from the bonds of religion and social custom, and brutally honest, but what he has given us is a man who has no inner life and who lives only by his instincts. While trying to depict a radically free individual, Camus has given us a man who is just like an animal, without any humanizing qualities. Meursault is not only unattractive, he is totally uninteresting. Who cares what happens to him?
I believe Camus made a mistake here. Based on his philosophy that the world is meaningless, he has depicted human life as meaningless, too. But he forgets the important point that human life can be given meaning by us - subjectively, when we make the effort - for example, by caring for others, by devotion to a job, by developing oneself, by interest in art and literature. "Art makes life, makes interest, makes importance," as Henry James said. It is up to us to discover the meaning of our lives. For, in this "absurd" universe, isn't consciousness a great wonder?