When a Lutheran looks at other confessions of the faith, he sees a matter of plus and minus.
On the Roman (and Orthodox) side of the equation, he sees the Sacred Scriptures plus. Stuff gets added to the witness of the Scriptures and those additions are also held to be normative. One thinks of certain theories of apostolic succession as a divinely mandated condition for the existence of the holy ministry; various teachings about the Blessed Virgin (immaculate conception and assumption); the invocation of the saints (as distinguished from the intercession of the saints), and so on.
On the "other Protestant" side of the equation, he sees stuff subtracted from the Scriptures: the saving efficacy of Baptism, the sacramental union of the bread and wine with our Lord's body and blood in the Supper, the authority of the pastor to forgive sins in Christ's name, the rejection of whatever is not explicit in the Scriptures (for example, the liturgy).
Lutheranism has always felt the tug both ways. We experience the temptation to add and to take away; plus and minus, if you will. The beauty of the Lutheran Symbols is that they don't give in to the temptations on either side. The same, sadly, cannot be said of us Lutheran pastors and parishes. We live in the dynamic tension between the two forces, and are always being allured to one side or the other. To teach more or to teach less than God has revealed to us in the Sacred Scriptures. The Symbols point a true media via. May God give us the grace to walk it - for the sake of the whole Church!