Prayer is defined in a Catholic catechism and in the Bible as raising the heart and mind to God. Jesus gave a top commandment to love the only true God totally with all your heart and mind and strength so that is implicitly a command to pray.
All prayer is a petition in the sense that you are asking God to reach down to you and help you raise your heart and mind to him.
The implication is that, if you pray to Lord Ganesh or Krishna or Marduk that is not prayer.
So Christian prayer is an offence against inclusion and religious equality.
The Bible strongly and violently opposes the worship as God of what is not God. For example, it forbids the adoration of idols. Nobody is misguided enough to worship images - it is either thought that the god is inside the image living or that the image has been transubstantiated into the god or that the image is just something you use to represent the god so that you can bow before it and so on and this obeisance goes to the being represented by the image. But it follows that if your religion that teaches you a particular kind of God is wrong you are worshiping a man-made creation when you worship that God. If man invented God you are honouring an idol. With all the lying religions going about, it is better not to take their Gods too seriously! The risk of idolatry is too great and would advise you to stay clear of these religions.
God being all good and all wise will not answer prayers that just request things from him without much concern for doing his will. This implies that if we want a bicycle, we should only want us because we feel it will help us serve God better. That takes the shine off religion! For a Catholic to pray that he or she will find a suitable marriage partner necessarily implies that you want a partner who will not use contraception.
Prayer is asking a supernatural power to do magic and do a miracle even if nobody notices the miracle. But belief in miracles is dangerous and incoherent and though it has been refuted conclusively centuries ago, Churchmen still persist in lying that miracles happen and that Jesus rose from the dead after three days and so on.
Prayer is based on an immature wish for God to treat you as special and leave the torments of life for others to endure.
Human persons assume there is a God just because they were lucky enough to have a fairly good and human life. This ignores the fact that there is more suffering and degradation and evil in the world than life and goodness. And especially when the endless evils that happen to animals are taken into account. It is the, "I am relatively okay in life and that shows me there is a loving God. I take comfort from that. I refuse to see that the suffering of animals and other people proves me wrong." Please don't take such an attitude. It is the same as, "my lover is kind to me and therefore he is a brilliant person who deserves the best out of life even though he is slaughtering babies every day of the week." It is really trying to get enjoyment while ignoring and refusing to emphasise properly with the suffering of other people and animals. It is admitting that if you suffered enough or saw enough evil it would prove to you that there is no God. But just because you are fine there is a God. That is really looking upon yourself as the most important thing in the universe. It is putting your own comfort first. Believers say that God allows suffering because of free will despite babies and animals suffering though they don't, according to the religions, have free will so they ignore this to believe in God. If evil doesn't disprove God then it follows that if I were the only grown-up in the universe and there were billions of babies suffering beyond belief all around me I may and perhaps should believe. If God exists and is good I am bad if I don't believe - an alarming thing to suggest.
To pray, "God bless you", or, "May God grant you healing", is really to mean, "God make that person take a terrible sickness and scream in agony for the next thirty years if that is your plan". Prayer and fanaticism are the same in principle. Prayer expresses the principle that religious doctrine and the God idea matter more than people. Christians are asked to approve of the killing and butchering that will take place when Jesus Christ returns to earth to vanquish all his enemies.
If prayer is to God, as Jesus decreed, then it is bad. Jesus said that even if you do all that God asks of you, you must still refuse praise and say you are a useless servant. This is false humility a deadly form of pride or it is a total absence of self-esteem. He didn't like pride so it was the latter he was advocating. Christian prayer calls for and asks for humility.
Christianity says you can pray for anything as long as it's not something that God has forbidden. Prayer needs to conform to the rule, "God your will be done." If it doesn't, it is really an attempt at black magick. In other words, you should only want the bicycle you are praying for to please God and nobody else. Prayer is impossible to square with human dignity.
If God sends frost and people die on the roads as a result of driving in it can you pray that he will take the frost away? The frost is an act of God. Asking implies you don't trust him very well and saying you don't approve of him sending the frost.
If prayer for another is praying that God will change you so that you are there for them and help them you clearly have a high opinion of your usefulness especially when it is somebody you don't know.
You pray for your friend's wellbeing. Instead something awful happens. If you care about them, you will consider blaming yourself for praying. If you attempt magical or paranormal means to help somebody, you have to consider that the disaster might be at least partly your fault. Taking credit just for the good is just arrogant. If you pray to be better at helping them and you end up hitting them then take responsibility for praying for something that went the opposite way! If you think all that matters is that you try to help, regardless of whether that helps goes wrong or not, you are inflating your own ego. It is not about the person helped.
Prayer resembles entitlement more than devotion. It becomes a way of placing yourself at the center of the universe: "Because I am okay, God must be good. Because I suffer, God must be testing me. Either way, I matter immensely." Incredibly, they say God does not want you to suffer so you are so important he has to let it happen anyway. God then torments himself for you to be better off than you are.
Finally:
If prayer does not work then you have conditioned yourself and others have conditioned you [you cannot condition yourself alone!!!] to think it does. That is dangerous. Help needs to be real or it is not help. Prayer needs to really help and not just give good feelings.
The Book of Atheist Spirituality, Andre Comte-Sponville, Bantam Press, Transworld Publishers, London, 2008