The first, and likely most important, is did President Trump ask James Comey to stop the Flynn investigation? If he did and if he fired Comey because Comey would not stop that investigation (two huge--absolutely HUGE--ifs), this situation rises above a political spat to being a Constitutional issue. Of course, these are only ifs right now--not established facts.
But there are other questions, too.
If the president's request occurred and constitutes obstruction of justice (again, if), why did Director Comey not resign and announce this request when it was made? Speaking purely hypothetically, if the president committed an impeachable offense, a government official would have an obligation to do all he could to ensure that this offense was known so that Congress could proceed with impeachment.
Have other government officials, including in the Obama administration, committed acts of obstruction of justice that Director Comey knows about but did not act on or announce to the world? What else could be revealed by reading Comey's private memos?
What is the journalistic justification for the New York Times publishing a story about a non-classified report that it has not seen? According to the story, sources only read portions of the memo over the phone. If mainstream newspapers want to distinguish themselves from tabloids, they will need to think hard about sourcing policies.
As many have suggested, the first step to answering some of these questions is for Congress to subpoena the Comey memos. That will help us distinguish facts from innuendo from lies. In a time when institutional trust is under assault, the rigorous attention to facts grows even more important.