From what I could see, you saw his vision, technique, elusiveness, decision-making and work rate on display. The most surprising element of his game to falter at this level was puck handling, but that can be attributed to nerves.
Other than that, his problem stemmed from what is key at the NHL level. You need to be able to give yourself room, whether by utilizing your size or by creating space by skating. He wasn't quite fast enough to buy himself time, so he got opponents in tight and was mismatched in those physical duels.
His primary area of development should be lower body strength, for balance and explosiveness both. If he can develop that over the year, I think next year he'll create himself the chance to utilize his vast talent.
Those that remember all the discussions leading up to the draft know that I wasn't necessarily in the Marner camp until late, but what I have seen so far indicates a player that need a year more (and we knew this) to add a crucial part to his game before he can translate to the pro level.