Schubert, like Beethoven, straddled the boundary between the Classical and Romantic eras, and thus brought some of each sensibility to his work. Schubert's works -- at least, the ones that I've heard! -- are pretty solidly Classical in their form, but the melodic subjects tend to be Romantic in feel, with long tunes and more chromaticism than you hear in Haydn or Mozart. This symphony is no exception: long tunes in long passages that occasionally seem almost "stuck", before subtly moving on in such a way that you don't even notice. Schubert can, at times, be almost hypnotic in the way he deploys dreamy melodies backed by ostinati.
Looking at background information on this work, again I'm struck by the fact that it was almost certainly not performed publicly until well after Schubert's death. He is one of the great, canonical examples of a great artist who was completely ignored while he was alive. I hate those stories.
The next four installments of this series may be a bit on the personal side. Reason? We're coming to the symphonies of Berlioz.