For me it's about whether the character is performing the function for which he was created: is he a hero, a stand-in for the reader, someone you aspire to be? Aaron's Thor is not, at least to me. You generally fine-tune a long-running hero to match what is considered 'cool' at the time of writing; Aaron's Thor is a patronising caricature of a Viking warrior, not an intended improvement on the Lee/Kirby model, imo.