Every master has a Master. There is only one God, one Lord, one Master. Though Jesus Christ came to this earth to serve and not to be served, Matthew 20:28, He still identified Himself as the disciples' Lord, John 13:12-17. They were always to recognize Him as Lord and Master of their lives. And that is what Paull is reminding earthly masters, or shall we say, employers. There's nothing wrong with being the boss. Someone has to. But, every boss needs to know they are accountable to the Lord, as we've already noted in Romans 14:12.
When Paul speaks of Masters treating their "slaves in the same way" he is speaking of treating them as Christ would treat them. And, furthermore, they should treat their employees the same way Christ treats the employer! The closer the employer is to walking with Christ the godlier and kinder he/she will be in dealing with their employees.
"Don't threaten them" speaks of abusive language to get them to work harder or work better. The Lord is still kind though He certainly can be tough! A master/employer doesn't have to yell at his workers. But he should speak to them in a straight forward way clearly explaining what is expected of them and what the results will be if they meet or don't meet those expectations.
"No favorites" with God? He treats the employees with the same love and kindness as He treats their employer. And, an employer should have the same attitude towards all of his employees. He should be kind and fair to all.