1. They are very demeaning to ChristiansOne thing you'll quickly notice about adament atheists is how demeaning they are to Christians. They will call them idiots, morons, and mock their beliefs with vitriol. You simply don't see this type of thing from the other way around though. Christians feel compelled to offer reasons for their beliefs and defend against atheism. They may say atheism is misguided, or even wrong, but they will not stoop to the level of calling them morons, dults, idiots, etc. for no reason. You may disagree, but I suggest you do a search. I'm sure in the whole world, there are Christians who are belittling and demeaning to atheists, but it's not the norm. Go to Youtube. Check out how many videos basically say that Christians are idiots. Then compare that to the rational response videos made by Christians. Even Richard Dawkins, sort of the global figurehead for atheism, calls his group the "brights", a clear antithesis to the "dulls" which he is implying Christians are.
2. They always seem angry
Atheists always seem kind of pissed off. They have loads of hatred and scorn. Check out for example Penn Jillette. He is part of the comedy routine Penn and Teller. He spews hatred, completely unbridled. Atheists always seem to have a chip on their shoulder.
3. Their "raison d'etre" seems to be to prove they are just as good as Christians morally
I've seen some pretty weak attempts by atheists to show they are just as morally good as Christians. They constantly have a need to prove themselves. But this attempt at overcompensation derives from their feelings of inferiority. Any atheist can stroll through any major city and notice that the vast majority of hospitals, schools, and other institutions of societal good come from people who believe in God. Faced with this stark realization, atheists go out of their way to prove they are "just as good".
4. They seem selfish
Because atheists see themselves as the greatest thing in the universe, they become rather selfish. Morals are self-derived. If something feels good to them or benefits them, then it's good, otherwise it's bad. If there is no God, humans are the highest form of existence, and atheists would place their own personal existence above others. Atheists do not want to sacrifice for the greater good, but rather want to revel in sensual pleasures.
5. They try to remove faith but it seems like a pointless activity
Why would atheists spend so much time and energy trying to eradicate faith. Mao, Stalin, and others did the same when they suppressed religion in the name of atheism, and we can see the terrible results there. If they believe people are following a "fairy tale" like the toothfairy or Santa Claus, then what harm is that? If these fairy tales makes the person more generous, kind, loving, and selfless, why bother removing this faith? Why don't atheists spend as much time promoting lack of belief in Santa Claus and the Easter Bunny to children as they do to eradicating faith?
6. Atheism makes the world a cold, heartless place
Atheism presents a very darwinian view of the world. Survival of the fittest. Atheists tend to attribute all their good fortunes to their own effort and see those they deem inferior to them as not as valuable. It becomes everyone for himself, and all this makes the world a cold and heartless place.
7. When everything is about science, people are reduced in value
Without our belief in a creator, atheists tend to view everything from the point of view of chemistry, biology, evolution, etc. People are no more special than a trout. They are just two different species. God didn't create us in a special way. We are all just accidents in a cold universe. Therefore those who are deemed "unsuitable" are no longer owed any special treat. The elderly, handicapped, and otherwise challenged individuals are easily seen as cosmic mistakes. This reduces the value of all humanity.
8. They lack in charity and use profanity
Atheists have no aversion to using profanity and vulgarity to make their point. Holding nothing sacred, they see blasphemy as a joke. Lacking charity, they feel they can express their opinion in any way they see fit, no matter how offensive. They see their behavior as "tough love" and denounce those who raise issues with their behavior as "softies".
Those are just some of the reasons atheists are no fun.