Jerkass Woobie: Grettir himself. He brings an awful lot of trouble on himself, but the reader really feels sorry for him.
Older Than They Think: When Thorbjorg the Stout finds Grettir captured by farmers and questions him, his response as to how he ended up there is essentially the punchline of the old "everybody gotta be someplace" gag.
Romantic Plot Tumor: The love story of Thorstein and Spes is a closed story arc in itself and has no relevance to the main plot at all.
Follow the Leader: The Norwegian horror story Glaam from 1925 borrowed the entity from the saga of Grettir. In this story, Glaam is a known harbinger of death, and is pure Nightmare Fuel. As in the original story, most people lose their wits, and their life, when they stare into his eyes.