I have been contemplating this question and my answer is, nothing. There is nothing inherently wrong, evil, etc with musical instruments but rather what man does with them.
For example, the church to which I belong uses instruments. While I don't believe there is anything wrong with that, the person that is our interim music director often times puts on a show. It comes across as more a concert than worship. We have had people walk out because it was too loud. As a comparison, the family went to see NewsBoys in concert, it was similar to our church's current worship style.
The instruments are not doing this but rather mans corruption. Of course my judging is just as bad.
The issue is whether God has commanded or authorized musical instruments in Christian worship, that is, if you care about what God has commanded or authorized (I realize most religious people do not really care, so this post is not aimed at them).
The nephews of Moses tried to worship God in a manner that he had not commanded. Scripture does not claim that they were told not to (forbidden). It was sufficient that God commanded a form of worship. The nephews of Moses instead devised their own worship.
Was God pleased?
"Now Nadab and Abihu, the sons of Aaron, took their respective firepans, and after putting fire in them, placed incense on it and offered strange fire before the Lord, which He had not commanded them. And fire came out from the presence of the Lord and consumed them, and they died before the Lord."
No, he was not pleased.
Do we think God is different today, that he will be pleased by worship he has not authorized, when he has commanded Christians to worship in a certain way, but they do what is pleasing in their own sight instead, and justify it with, "I see nothing wrong with it."
What did God speak about the nephews of Moses after the event, to instruct Aaron, their father, the high priest, so that he could instruct the people about this event?
"By those who come near Me I will be treated as holy,
And before all the people I will be honored."
In other words, offering worship that God has not commanded does not treat him as holy (exalted or worthy of complete devotion) and does not honor him before the people.
This incident with the nephews of Moses was written for our learning, recorded and preserved through God's providence for thousands of years so that you can read it and learn from it today.
"For whatever was written in earlier times was written for our instruction . . . "
What is the "instruction" that it gives?
Answer: To treat God as holy and to honor him, worship in the way he has commanded only.
Claiming that your favorite practice of innovative worship, whatever it is, is not "forbidden" is not sufficient to justify the practice. God will be treated as holy by those who come near him.