So good points this far that I'm almost afraid of putting my shallow scramblings here...
I very much agree with Child that Frodo's not a person who lacks character, though I think it's very easy to think so. Frodo is always a bit distant character to the reader, but the more I read LotR, the more I understand and admire him. He could nearly fit into an old ancient Greek tragedy.
Frodo's personality may seem something straightforwardly protagonist heroish. I agree to some point; I think the biggest problem with his character is the lack of faults and vices. That leads to that if the book is not carefully read, his character lacks nuances. I think anybody who thinks Frod's lacking character should read through the Bree-chapters. All that jumping on the table and singing...
Though I think Frodo has a personality of his own I still agree with Nogrod that he's one of the "thinnest" of the main characters. Partly restating Wiscott here, I think Frodo is, as well as the sacrifice
in the story, the sacrifice
for the story. His "general hero personality" works well to emphasise other characters' personal qualities. Also, he represents the Ring's power and the corruption it brings,; he represents fading (like the Elves) and putting other people before himself. A person with such a quality must seem very strange indeed; seldom we see people who can sacrifice themselves for others.
P.S. I'm aware of only agreeing and flip-flopping here - must have learned that from werewolf!