First, I don't think ASM1 was 'dark and gritty'. The highschool fight with Lizard was the most upbeat, lighthearted action sequence I can think of. Lizard wasn't intimidating, he was a joke. The fight itself was littered with jokes, from webbing Lizard's mouth to the Stan Lee cameo.
Second, a good Spider-man movie should have a variety of tones. When appropriate, there should be comedy, horror, tragedy, suspense, romance, drama etc. Parker/spider-man needs to be isolated to a degree, and secretive because of his identity. He should have an air of introversion and loneliness, he talks to himself a lot, he spends a lot of time alone in contemplation. He's pessimistic and complains a lot, yet he has a sarcastic wit and sense of humor which he uses to make light of serious situations. He can be arrogant, he'll compliment and promote himself shamelessly but he's also a very guilty person who tortures himself over his mistakes.