The first misconception shows itself in believers who, while they pay lip service to a gospel of grace, live their lives under a non-gospel of works or law. They believe that in order to keep God's favor, they must live a morally pure life. When they sin, believing they have lost their good standing before God, they try to avoid God until they have cleaned themselves back up. What makes this non-gospel so devastating is that it prevents people from doing the one thing they should be doing when they sin, namely running to God. Instead, they run from God in the shame and condemnation of their sin. These people are typically frustrated and worn out from trying to keep God's approval (which in reality they already have through the blood of Jesus shed on the cross.) They miss the joy of obeying Christ because they have tied their obedience to their righteousness before God.
The second misconception occurs when we take the "not by works" aspect of the gospel to the point that we do not feel the need to obey God at all. In the effort to avoid legalism, apathy and inaction instead occur. These people are often quick to cry legalism at the first hint of the topic of obedience to Christ and His word. These people also miss out on the joy of following Christ because they aren't really following Him at all. They are just drifting through life with no real purpose.
A right understanding of the gospel frees us from working to attain God's approval while also freeing us to work for the advancement of His kingdom. When we know that our righteousness is solely dependent upon the work that Christ accomplished on the cross, then we don't need to constantly fear condemnation due to our own sins. We can run to God in repentance when we sin knowing that He will never cast us out, knowing that Jesus has secured our place for us. Being secure before God leaves us unafraid to fail in our pursuit of obedience. We can, through the power of grace, obey the Word and know that when we fail, the blood of Jesus covers us completely.
So the same gospel that frees us from work also frees us to obedience. It is in our growth in obedience, failures and all, that we experience the true joy of following Christ. As we enjoy the fruits and trials of obedience and the pain and discipline of disobedience, God grows us closer to Himself. It is, of course, as we grow closer to God that we begin to understand what true joy is.
But we will never experience that joy until we truly understand the nature of the gospel.