Symeon writes about his own experience in the first person, in detail, which almost nobody in the Byzantine tradition did. He describes a light that is not a metaphor and not a symbol, filling him, addressing him, leaving him weeping and unable to say afterwards whether he had a body at the time. His Hymns of Divine Love are extraordinary in their directness, and in places their erotic frankness about union with God embarrassed later copyists.
The doctrine he built on it is the reason he was a problem. Symeon held that the conscious experience of the Holy Spirit is not an extra given to a few specialists in the ancient past but the normal condition of a Christian life, that a person who has not had it should say so honestly rather than assume it happened invisibly at baptism, and that anyone who denies it is possible now is calling the promises empty. He preached this to a metropolitan monastic audience, and it was heard as an accusation, which is roughly what it was.
His nickname, the New Theologian, was probably not a compliment when it was first used. The Byzantine church reserved the title theologian for John the Evangelist and Gregory of Nazianzus, and applying it to a contemporary abbot suggested pretension. It stuck, and has come to mean what it says.