One thing which the confessions have in common, although it is particularly pronounced in the earlier confessions for obvious historical reasons, is opposition to Roman Catholicism. "In the bitterness and disgust with which they speak of the pope and the mass, there is scarcely any notable difference between the Swiss and the German, the Eastern and the Western confessions". Barth argues that this opposition to Romanism is more deliberate, and more central, to the Reformed confessions than to the Lutheran, although all Protestant confessions carry the awareness that Rome is the undoubted enemy.
Barth sees two subtly different positions at work here. The Lutherans are cross with Rome because Rome robs Christians of assurance and despoils Christendom (thus dishonouring God); the Reformed are cross with Rome because Rome dishonours God (by robbing Christians of assurance etc). The Lutheran confessions stress salvation by faith in Christ; the Reformed confessions stress salvation by Christ through faith. The Lutheran position is pastoral first; the Reformed position is theological first.
Now, I don't know how accurate a portrait that is of Lutheranism, but I think it captures Reformed concerns perfectly. The point is that, contra Rome, the Reformed maintain that God does everything. Hence the typical (almost stereotypical) Reformed concern for the sovereignty of God, expressed in the doctrine of predestination.
Here is theology I can get behind!