This is very much early Miyazaki. Early in the sense that he seems to have difficulty trusting the audience, instead ensuring that characters tell us what they are seeing and explain it - twice if necessary - so that there is little room for misinterpreting the potentially confusing scenario. Miyazaki in the sense that it is fully committed to its environmental message in a way that manages to be simple without being simplistic.
Like many environmentally concious films it's about being able to live in harmony with nature and respect it rather than struggling against it. Unlike many such films its message is never presented in such direct, cliched tones and the destructive impulse to be struggled against is not as simple as the commonly trotted out sin of greed. Rather than a parable of avaricious humans destroying the natural world out a myopic desire to enrich themselves it's about people trying to make life comfortable for themselves without understanding the environment they're manipulating. It's harder to boo and hiss at the bad guy while remaining confident of one's own good nature when everyone is acting out of recognisably common impulses.
It's also a pacifist film - as has been noted elsewhere much of the narrative is driven by Nausicaä's desire to atone for her early killings by attempting to resolve conflict without bloodshed. As with the environmental message Miyazaki avoids the easier strategies of polemic. While the hero is (of course) triumphant, the narrative is not naive about the possible consequences of attempting to face up to armed soldiers with bare hands and honest words.
The earlier noted combination of early difficulties and precocious assurance is repeated with the visuals. The animation, in keeping with the lower budgeted productions of Japan at the time, is often minimal and occaisionally awkward. However the art is as lush as ever and one can see, in the design of creatures, people and craft, many of the same aesthetic ideas pursued in later Ghibli films.
While uneven it's the strengths of Nausicaä that linger longer than the weaknesses. Once the audience is comfortably orientated in his world Miyazaki does let the story flow more smoothly with less intrusive hand holding and the story's strengths and even some of his humour emerge.