It's really not as simple as that. There is a lot of ways to look at sin, but probably the best way for discernment of differences is to consider the consequences of sin.
Some say that all sin is equal. But this is only true for those who are under the law. For those of us who are saved by the blood of Christ, and whose sins are propitiated before the Father by our High Priest, this is not so.
Once you are saved, then Jesus has taken upon Himself the eternal consequences of sin. Thus, if you believe in "once saved, always saved", then you need not worry any more about the eternal consequences. Even if you believe as I do, that you can still lose your salvation, you must completely turn your back on Jesus, and deny the Holy Spirit, in order to do so.
As such, a greater impact for sin is experienced on a temporal level, therefore in the quality of our lives and our relationship with Christ in the church. This being so, it would be hard to argue that cutting someone off on the highway, or uttering a curse word, is equivalent to adultery. However, we must also consider the effect of our sins as stumbling blocks to the unsaved as well.
Paul recognized that certain sins had greater corrupting influences in our families and church and needed to be dealt with agressively. These sins included drunkeness, covetousness, idolatry, reviling, swindling, and the sexual sins such as fornication, adultery, and homosexuality. I don't exactly know why Paul was more concerned about these sins than he was others, but I trust his judgment.
I believe, to a greater extent than other sins, that these types of sins tend to divide families and the church, separate us spiritually from Christ, and to retard the churches ability to evangelize the lost. They also hinder our growth in the faith, and the ability of God to use us for his purposes. Paul lists these sins in 1 Corinthians 5 and he considers them great enough to expel those from the church who don't repent of them.