"A church is an assembly of baptized believers, joined by a covenant of discipline and witness, who meet together regularly under the preaching of the Word of God."
So what does this look like ideally? Obviously this is going to be opinionated. Also, I guess I should insert American or something since assemblies of believers will look different in different parts of the world at different times. However some of these principles will actually be the same wherever and whenever you see them even though the outward manifestation may not appear congruent.
While a church is not a building (according to our working definition) the place of assembly should look like a place set apart for the worship of God. It should not look like something that got thrown up overnight or like something that will fall down today. It should not be dwarfed by a Family Life Center, a bookstore, a school, a bus depot or other extracurricular structures. As you enter this building it should have a sanctuary that calls for a solemnity and consideration of the greatness of God. [Aside: I think that one of the huge problems of modern American Christianity is that we have allowed our churches to become multi-purpose theaters where many things take place and consequently our worship just become one among the many mundane things and is no longer holy and special. But I will take that up another time.]
The people that we see gathered should look as though they came here on purpose with preparation to do something that is unlike other mundane gatherings throughout the week. I do not espouse a certain kind of dress I simply say that they should dress in a way that is set apart, that indicates a reverence and desire to worship. As we look around at those assembled it should not look like a crowd at a football game or movie theater. It should look different because they are here for a different reason. The conversations overheard should be different than the everyday water-cooler fare. These people should be engaging in each other's lives and not simply rehashing the weather, the workweek and the latest TV show craze.
As the service starts, one would hope to observe in the liturgy (music, prayer, reading, speaking) a formality without formulaity that indicates that this is not just a political meeting or hobby club but a group that is coming before the Creator and Sustainer for the purpose of hallowing His Name. If the music is just like what you listen to while passing time on your commute then there is a problem for it should be more thought-provoking and stirring. The prayers should not sound like your dinner prayer they should be intoned and addressed as being from the entire group to God and they should stir up truth as we listen to the one praying. As we read the Word and it should be done with worshipful excellence not hurried through or stumbled through. It should not be done in antiquated language that is incomprehensible to the listeners. [Aside again: I always find it strange when preachers who never say "Thou" when talking to you will say it 17 times while preaching. What is that about?]
Lastly, the church has ordinances. We were given them for a reason and we should use them to accomplish the purpose for which they were intended. That is a whole topic in itself but suffice it to say that I believe we have taken these wonderful pictures that Christ gave to us and cheapened them almost to the point of insignificance. The ideal church should be making these ordinances a centerpiece of their mission to discipline each other to holiness and witness to the world of the greatness of God.
American Christianity is made up of many churches who do not have a clear purpose. They go through rituals without understanding the purpose and basis of what they do. Of course I do not believe that all or even most of the churches are completely lost but this is the kind of thing that creeps in on an assembly and starts to slowly choke out life. Thus it is incumbent on those that want to continue, while the Lord tarries, to see their bodies grow and accomplish the Great Commission to actively fight to maintain an assembly of baptized believers, joined by a covenant of discipline and witness, who meet together regularly under the preaching of the Word of God.