It was about serious stuff and the ultimate issue is whether Trump, or someone at his behest, told Flynn to call the Russians and reassure them we were going to do them some favors. That is the question which Flynn has never answered. Who told him to call the Russian ambassador Sergey Kislyak, to talk about Russian interests and upcoming vote at the UN, in which we interceded to help Russia? Where did that order originate? Flynn did not do that on his own. It is a specific thing we did and there were multiple calls to discuss it.
That is the EXTREMELY CRITICAL but UNCOMFORTABLE question for Trump that lingers and which will likely tell us, once and for all, whether there was a quid pro quo. Trump most assuredly hopes that the pardon will prevent Flynn from ever disclosing that.
But I think we all know the answer. And hopefully within Trump's lifetime we find out if it was traceable back to him.