This one almost goes without saying, but it's essential Amazon give this the money it needs. That should happen, since they've paid so much just for the rights, but they have little experience so far in working with the kind of budget this show would require.
Peter Jackson's three Lord of the Rings movies each had a budget just shy of $100m, while that's roughly around the same budget allocated for a season of Game of Thrones (and Netflix shows like The Crown and The Get Down are, by some reports, even more expensive).
A big budget doesn't guarantee quality, but if you're wanting to do justice to Middle-earth then you need absolutely lavish production design, lots of extras, some big action sequences, and probably a decent amount of CGI as well. The Man in the High Castle is the biggest show they've done so far, and Lord of the Rings needs to go much bigger than that - and it'll be interesting to see how Amazon handles the challenge.