I've only watched Hellsing Ultimate. The Major recognizes Seras as a real threat when he warns Zorin before attacking Hellsing's manor. He calls her a miracle of sorts.
Sure enough, she's clearly stronger than most other vampires. But why? Was it simply the fact that Alucard himself transformed her? What made her special if she didn't even consume Alucard's blood? Why does the Major think so highly of her?