Bruckner always put great emphasis on form and structure. His themes are often somewhat schematically based on triads, semitones and octaves. These themes almost seem like they write themselves, without much human in(ter)vention. I feel that's a strong classicist element in him.
His approach became even more severe after 1875-77, while he was working on his Fifth. But take a look at the original versions of his earlier symphonies. To me, they feel less restraint and more romantic, if you like, than the revised versions.
Post 1875-77, Bruckner found his definitive style, but it's surely not for everybody. I always wonder why Bruckner and Mahler are often mentioned in one breath. At the heart of it - and this is perhaps what principe was referring to - they couldn't be more different.