Church etiquette varies greatly between the different nations and cultural groups among whom the Christian Church is found. In Western Culture, in common with most social situations, church etiquette has generally changed greatly over the last half-century or more, becoming much less formal. Church etiquette might be seen to mirror other social changes, with the use of given names for leaders, informal dress.
In North America and Europe, up until the late 1950s it was often expected that worshippers wore their best clothes to church services (the Sunday best as it was known colloquially). This tradition has declined in many mainstream churches but is still much in evidence in the Southern Baptist tradition in the U.S., and in many black evangelical churches.
Those who support more relaxed dress codes do so on the basis that we should come to God as we are, and that communion with God requires no special clothing. Those who support more formal dress consider that although communion with God does indeed not require special clothing, a church service is an office of devotion and it is therefore appropriate to wear your best clothes.
Even where dress code is more relaxed it is still generally considered proper to dress modestly.
It is generally considered poor form to consume alcohol (other than as part of the sacrament) on church premises, and turning up intoxicated to a church service would generally be considered disrespectful. The Methodist tradition goes further, frowning on alcohol altogether.