Firstly, I must clarify that I'm not saying it's an objective 10/10. The "nostalgia glasses" are heavy on my nose, because it's one of the first animes I've ever watched as a kid. This, along with Lupin, Grendizer and Gundam were my father's childhood shows and I'm very glad he shared them with me.
Having said that, this work is a journey that is truly an ode to friendship and love. It sounds pretty generic, but trust me: this is one of the best depictions of love in fiction. The protagonist and his love interest's feelings are shown through normal events and actions, not through obnoxious lovey-dovey smoochy nonsense. I think they kiss only once or twice in the entire series. This makes their relationship very enjoyable.
The cast of characters is very diverse and enjoyable. You can find reasons for the way they act even in the most unlikable ones. There's not much in the terms of character development, except for a specific character, but i don't think it's a flaw. Having a "deep character development" (a phrase wich is overused by many anime fans) is not a requirement of a great story.
However, this is not only a love/friendship story. This is peak dystopian sci-fi with a great focus on environmentalism, impressive for a 1978 series. It gives an urge to deepen our contact with nature and manual labor, to create rather than destroy. My father really enjoyed it along with the unsubtle historical references, since farming and history are his greatest passions (his favorite scene of The Gladiator is the wheat field scene, even though he didn't watch the movie).
In the visual department, this anime has a simple yet effective mecha design, in line with other Miyazaki and Otsuka works. The stars of the show are its landscapes: islands, ruined futuristic cities, deserts, mountains, caves, the sea; all tastefully drawn. Speaking of how the series looks, there are very few recycled animations, and they don't annoy me at all.
Mirai Shounen Conan, as far as I remember, doesn't have many unique OSTs, but they are very enjoyable and give a nice feeling to the setting. There are also moments of silence, where only the sound of the wind fills the viewer's ears, creating a very nice vibe.
To sum up, I definitely recommend this anime to everyone that calls theirself a Miyazaki fan. He's done more than the movies, like this anime and some episodes of Lupin and Sherlock Hound. This is a series that still inspires my ideals and what I hold dear in life. As long as your anime taste goes beyond battle-shonens, isekai and slice of life, you will definitely like this.

(I really don't know what more to include, so these lines are just a futile attempt to reach the minimum words required)