What is wrong with Christians and Christianity? ?

Nothing innately. That said, many who claim to be Christians and claim to act in the name of Christianity have deviated so greatly from what the actual teachings of Jesus Christ were, such that they very often teach and do the exact opposite of what Christ taught, that they are not actually Christians, but Christians in name only. 

Jesus Christ, for example, taught us to love one another and do unto others as  we would have them do unto us, taught us to feed the hungry, clothe the naked, shelter the homeless, tend to the sick, take in foreigners, etc., but how often do we hear self-proclaims Christians rally vehemently against us feeding the hungry, housing the homeless, providing healthcare to those who are sick, taking in foreigners? In America, the Christian fundamentalists are the driving force behind eliminating food stamps for the poor, WIC providing food and nourishment to young mothers and children, HUD providing housing for the homeless, any national healthcare program providing access to healthcare to all of our sick, and going so far as to build a wall to keep foreigners out. They say they're Christian, but where the rubber meets the road, they are anything but Christian. 

For Christ was an hungred, and they gave him not meat; thirsty, and they gave him not drink: Christ was a stranger, and they took him not in: naked, and they clothed him not: Christ was sick, and they tended him not: Christ was in prison, and they came not unto him. 

And these so-called Christians may ask, When saw we Christ an hungred, and fed him not? Or thirsty, and gave him not drink? When saw we him a stranger, and took him not in? Or naked, and clothed him not? Or when saw we him sick, or in prison, and tended unto not?

And the Jesus shall answer them and say unto them, Verily I say unto you, Inasmuch as ye have done it unto one of the least of these my brethren, ye have done it unto me. Then shall he say also unto them, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels. For I was an hungred, and ye gave me no meat: I was thirsty, and ye gave me no drink: I was a stranger, and ye took me not in: naked, and ye clothed me not: sick, and in prison, and ye tended me not.

Hypocrites ********

There are some things "wrong" with every human being, since no human being is perfect. However, the teaching of the true Christian Church is perfect, because its founder, Jesus Christ, Who clearly demonstrated that He was God, promised its leadership "The Holy Spirit will guide you into all truth", and "Whatsoever you bind upon Earth is bound in Heaven", and "He who hears you hears Me".

They've been duped into believing in superstitious nonsense.

It's not their fault, most were being taught to believe in it from before they even knew what they were being told.

All are sinners.

It is foolish of you to pretend you are any better.

Your question is very broad. Genuine Christians are those whose daily lives are centered around Jesus' teachings and principles of the Bible. Christianity has as its center Matthew 22:36-39 - “Teacher, which is the greatest commandment in the Law?” 37 He said to him: “‘You must love Jehovah your God with your whole heart and with your whole soul and with your whole mind.’ 38 This is the greatest and first commandment. 39 The second, like it, is this: ‘You must love your neighbor as yourself.’

People often look with contempt on Christians who endeavor to carry about the above verses by means of their Bible preaching and teaching (Matthew 24:14; Matthew 28:19,20) as Jesus commanded.

It's a nonsense belief system that is based in hatred and bigotry.

Nothing, it's the atheists who think about gay sex that is wrong

The truth is that death leads to heaven or hell, and the only way to heaven is through faith in Jesus Christ.

The Bible is very clear that no one can earn their way to heaven since good works do not pay for sins. See Gal. 2:16, Rom. 3:23-28, 4:4-5, Eph. 2:8-9, Tit. 3:5. The only way to heaven is by believing in Jesus for His free gift of eternal life. Jesus shed His blood on the cross as the full payment for our sins. He did all the work; all He requires of us is that we believe in Him. Once you believe in Jesus Christ to save you, you are secure forever!

Because we have all sinned against a perfectly holy and righteous God, we all deserve eternal separation from God in hell (Rom. 6:23). However, God loves us so much that He sent His only Son Jesus to die on the cross as our Substitute so we don't have to go to hell. Christ died for our sins, was buried, and rose from the grave three days later (1 Cor. 15:3-4). He gives eternal life freely to anyone who simply believes in Him for it. No one can be saved by doing good works. Salvation is by grace alone through faith alone in Christ alone (Eph. 2:8-9).

"Verily, verily, I say unto you, He that believeth on me hath everlasting life" (John 6:47).

All Christians are sinners. Forgiven sinners, yes, but no Christian can achieve, let alone maintain, the perfect standards of holiness Jesus set us. We try but someone can always fling accusations at us that will be at least partially true. As for Christianity - well, that's a name for the collective organization of Christian faith over the centuries. It's been full of error and awful things done by hypocrites so the world will always find the mud it flings sticking. But God didn't sent an organization to Earth to save it - He sent His unique Son. Trouble is all individual Christians and collective organizations take their eyes off Jesus.