There are, IMO, four things that so far separate Tannehill from Mariota.
1. Passing accuracy, velocity, and ability to anticipate leading receivers resulting in greater runs-after-receptions.
2. Quick releases, thus reducing the effect of mediocre OL blocking.
3. Willingness to throw into tight windows.
4. Command of the huddle and holding teammates accountable.
I added #4 because Mariota seemed to always accept blame when it wasn't always his fault. It is hard to know if MM did hold individuals responsible, but that simply does not seem to fit him.