I do not believe that the largest expression of unacceptable worship is not whether one worships with the correct form, or uses some sort of innovation, like a musical instrument. It has more to do with what is in a person's heart. In Luke 18:9-14, Jesus tells a story about two people that went to the temple to pray. One was a Pharisee who stood proudly before God as he prayed to him. This Pharisee was obviously proud of his religiosity. He performed all of his religious duties flawlessly. On the other hand, a tax collector stayed at a distance and did not even lift up his eyes. Beating his breast, he cried out to God for mercy. Jesus tells us that the tax collector was the one who left worship justified that day, not the Pharisee. How could that be? Wasn't the Pharisee a good man? Didn't the Pharisee perform all his religious duties? Obviously, there is more to worship than performing religious duties.
After David had sinned with Bathsheba, he writes,
"For You do not delight in sacrifice, otherwise I would give it; You are not pleased with burnt offering. The sacrifices of God are a broken spirit; A broken and a contrite heart, O God, You will not despise." (Psalm 51:16-17)
What this tells us is that God is not interested in religious rituals. David understood this. David didn't just ask for forgiveness, nor did he merely ask to escape God's wrath. Instead, he asked,
"Create in me a clean heart, O God, And renew a steadfast spirit within me." (Ps 51:10)
David knew that God wants a pure heart, not religious rituals. David understood that God wants a relationship, not a day of the week. David was a man after God's own heart, and therefore did not have a warped view of God.
I believe that ones view of God will shape the way one worships. For example, if you see God as a vending machine, you will concentrate on putting in the right amount of money and pushing the right buttons to get what you want. In other words, saying the right words in prayers, performing the right rituals, giving an offering, etc. But that is not who God is. Acceptable worship is not merely about pushing the right buttons on Sunday morning. It is not about going through the right motions at the right time. It is not about taking communion every Sunday as if you were punching a time card. It is about having a pure, loyal, devoted heart both in formal worship and throughout the week.