At its core there is a defensible part of The Donald's decision on immigration and refugees: that there be a freeze on entrants from some parts of the world or certain countries due to the conditions there.
At the same time there are multiple levels where what he did is objectionable.
1) Administratively. This was rushed through apparently without the parts of government responsible for its implementation having a chance to review it. The result has been chaos. And possibly tragedy and suffering, as for example the case of the Christian family that landed in Philadelphia but ended up taking a plane back to Qatar.
2) Policy details. It should have been made clear from the start that this did not apply to legal permanent residents. I also happen to believe that the vetting procedures for the seven affected countries are already sufficiently rigorous, but this is something reasonable people can disagree on.
3) Morally. I don't think this is included in the executive order, but Trump has said he specifically wants to help Christians in Syria. Now, there are circumstances where it is morally right to focus your efforts to help specific religious minorities who are being persecuted. But the situation in Syria is one where all groups are suffering, more or less equally. In those circumstances, I think it is morally wrong to single out one group as the one you are going to focus your efforts on.