This doesn't seem to be a very difficult question but from what I gather, the answer many give is made of men's traditions and not what we see in scripture. Even those who believe that the New Testament churches only serve to describe the church gatherings and are not to be considered as a blueprint for what the church should look like today often pick and choose traditions that are based on a handful of out of context verses that they think should be followed like a law. It is very inconsistent to say the least.
I recently watched a video from the Gospel Coalition with the same title as this post. It was long so I mostly skimmed through it just to get an idea of what they were talking about. According to these men, the local church is mostly defined as a place where the scriptures are preached in an expository manner, there is a focus on making disciples and they have proper guidelines for membership status. Really??? I keep looking for the passages that describe the church congregation or elders voting someone into the church and I can't find it. Maybe someone could help me with that. Likewise I haven't found the passages that show a leader preaching a sermon to a body of believers. If these are the marks of a healthy church then the first century churches that Paul planted were not worthy of imitating.
Why would we want to be like the first century church anyway though? It's not like they were the picture perfect group of folks that were sold out for Jesus. There was fighting over their favorite leaders, bickering amongst the women, show-offs dominating the gatherings, greedy people eating up all the food before everyone else arrived, some were teaching that real followers of Christ had to be circumcised to prove it. I mean really, these folks were jacked up. Surely we have matured a lot since the first century as a church, right? Why would we give up our comfy pews where we listen to a skilled communicator and teacher break down God's Word for us? And...don't our modern membership guidelines keep the jacked up people out of our congregation and make sure everyone is on the same doctrinal page?
Not exactly...
So, what does the local church look like? If it's a biblical church it will look very similar to what we see in the New Testament. It will be full of folks who have a lot of room to grow. They may even be as jacked up as the ones we read about in the Epistles. Yet, they will have one thing in common: faith in Christ. They will need to be constantly reminded to focus their gaze and minds on Him. They will give of themselves, of their time and resources in an effort to build others up by pointing them in the direction of Christ. They will depend completely on the indwelling power of Christ's Spirit to bring about their growth and they will boast only in their weaknesses.
What doesn't look like a local church? The opposite of the above. It will look drastically different from what we see in the New Testament. It will be full of folks who think they have it all together. They will give the impression that everything is just hunky-dory in their daily life. They will have one thing in common: a church covenant. They will keep themselves very busy with programs and church functions so that they are not distracted to commit the most heinous sins. They will come to church and be a part of the programs that benefit them the most. They will depend completely in the right teaching of their preaching pastor and Sunday school teacher to bring about their growth and they will boast in how much they love their pastor and church because they are doing everything right.
If you're looking for the 9 marks of a healthy church you can start by looking for the marks on their knees from prayer, the marks on their hands from service, the marks on their Bible from persistent study, the marks of their past where the Lord has corrected them, the marks they have made on the lives of those they come into contact with, the mark of humility, the mark of joy and the mark of peace that comes from walking with the Lord everyday but most of all look for the marks of the Spirit living through them to love, serve, grow and bring others to a saving knowledge of Jesus Christ.
Sorry to bust your bubble but expository preaching, evangelism, and strict membership guidelines including a voting process for new members is not what makes a church. It certainly doesn't mark a healthy church.
A flesh and blood body of Jesus Christ living by His Spirit is what makes a church. Period.