This is a legal/political question - though, I don't think the political aspect is divisive enough to belong in the PSF. (i.e. think about this as a lawyer)
So, by now, we all know that Barr has concluded that the evidence of obstruction against Trump is insufficient to warrant charges - i.e. DOJ does not think they could prove the elements of the crime beyond a reasonable doubt. Without seeing the underlying evidence, I still think Barr is probably correct here, and from a legal standpoint is on solid ground.
My question for the bright legal minds here, is: "Should Barr have issued his opinion?"
The relevant Special Counsel Regulations:
(a) The Attorney General will notify the Chairman and Ranking Minority Member of the Judiciary Committees of each House of Congress, with an explanation for each action
(1) Upon appointing a Special Counsel;
(2) Upon removing any Special Counsel; and
(3) Upon conclusion of the Special Counsels investigation, including, to the extent consistent with applicable law, a description and explanation of instances (if any) in which the Attorney General concluded that a proposed action by a Special Counsel was so inappropriate or unwarranted under established Departmental practices that it should not be pursued.
My read of the Regulations is that Barr was not required to issue his opinion on the validity of an Obstruction of Justice charge. The regulations state that the report has to explain each action taken by the SC - and, in this case the "action" taken by the SC was to not decide on the Obstruction of Justice charge - and that should have been, and was, reported by Barr.
In addition to the above regulations, there is long-standing policy at DOJ to not indict a sitting president, rather deferring to the constitutional process of impeachment. In his letter, Barr said he reached his decision, notwithstanding that long-standing policy. But, I think, that by issuing his opinion, he was stepping over that policy, and into the authority of congress.
In this instance - Barr was reaching a conclusion, that he was not required to make. Much like a court should not decide an issue that is not before it - I think it was improper for Barr to issue his opinion on the underlying crime. DOJ was not going to indict the president, even if Barr reached a contrary conclusion. The question of Obstruction of Justice (and even the conspiracy charges related specifically to Trump), is a question that the constitution says should be answered by Congress.
Now, from a legal standpoint, Congress is still free to make these determinations once they get the underlying reports/evidence. But, has Barr impermissibly injected his own opinion into the public discourse here - thereby influencing public opinion on a political question?
I think the answer is Yes - Barr has overstepped his bounds here, and ethically he should have remained on the sidelines (in terms of offering his legal opinion to a political question), unless Congress asked for his opinion.