When I think of the briefest way to characterize the requirements of "Called to Common Mission" (CCM), it goes like this. CCM requires both the ELCA and the Episcopal Church to make concessions to the other church body for the sake of full communion. Following the Lutheran Confessions, Lutherans have always held that all that is essential for the church to be the church is the presence of God through the gospel and the sacraments; they have also said that this is all that is necessary for church unity.
Of course, Lutherans realize that certain organizational, constitutional, and financial aspects also will be involved, but these things aren't necessary in the same sense that the gospel and the sacraments are, nor is any single form of such secondary things to be required for the sake of unity.
The Episcopal Church (ECUSA), on the other hand, agrees that the gospel and the sacraments are essential and that they are sufficient for salvation and for a church to be recognized as a church, but (Episcopalians say) they are not enough for the unity of the church. For unity, they hold, bishops in historic succession are necessary (and, likewise, pastors/priests).
In CCM the ECUSA makes the concession of accepting present ELCA bishops and pastors as being available for ministry in the Episcopal Church for as long as it takes until all ELCA bishops and pastors have been properly ordained (40? years of "suspension of the Ordinal") The ELCA makes the concession for the sake of the Episcopalians of agreeing that it will invariably install ELCA bishops into the historic episcopate and that these bishops will ordain all new ELCA pastors.
In other words, the ELCA agrees to require this and practice it although it does not believe it is necessary, even for church unity, as a concession to the ECUSA. ELCA opponents of CCM think it is wrong to require something that Lutherans explicitly do not believe to be necessary; further, opponents believe that their opposition is a matter of confessional commitment, not merely private opinion or whim.
On the other hand, some Lutheran supporters of CCM have suggested that this is really not all that important and "it won't change anything." More careful CCM supporters in the ELCA argue that as long as both parties agree on the gospel and the sacraments-and agree to move forward on that basis alone-then all sorts of practices can be required that will make that unity more effective.
Opponents are very skeptical of this claim. The ECUSA might add that the ELCA is not the only one giving up something. Never before has an Anglican Church body suspended the Ordinal in this way and some Episcopalians find this concession to be very questionable. However, it has not generated any sizable conflict in the ECUSA. The debate and division are largely within the ELCA.
A major part of the problem has to do with the asymmetry of the concessions. The ECUSA makes a temporary concession that will involve only a small portion of ELCA pastors who happen to be invited to serve in Episcopal congregations. The ELCA concession, on the other hand, is permanent and will involve all new bishops and new pastors from now on and forevermore.
It is the fear of opponents of CCM that this change of practice will involve raising something that Lutherans do not believe to be essential (episcopate) to the same level as that which is essential (gospel/sacraments). That seems to some Lutherans to be what has happened in the ECUSA, sometimes to the detriment of preaching and teaching-including the loss of much confessional loyalty to the Anglican Thirty-nine Articles.
To many Lutherans CCM seems to compromise the freedom that Christians receive when they are justified by Christ through faith and to limit the church's creative flexibility for mission that follows from this central teaching of the Lutheran confessional witness. This is why the struggle within the ELCA is so intense.