I didn't think he was THAT bad, but he sorta lacked an attitude. I think it was that weird accent that he had that made him sound off...teamed with his very childlike attitude. Lucas has a problem of making children act like they're 5 years younger than they are, especially if they're supposed to be really serious even as teenagers.
Anakin should not have said "yippee!"
Boba shouldn't have said "fiyuhl" instead of "fire!"
I think Boba would have been an awesome character if he'd have been portrayed as a sick little kid, like, well take that line. He could have said something real demented for a kid to say, like "Kill him Father!" and could've had a genuinely sick and twisted look on his face as if he's REALLY enjoying it, then could've had the anxious face like "FIRE!!!" sorta like Kill him dad, I really want to see him die.
Anakin should've been a more serious child. He shouldn't have been saying the stupid crap he did. He should've been more along the lines of being the child who thinks he knows everything and will fight against a 50 year old if he doesn't agree - Episode II Anakin.
If we had had Episode II Anakin's method of thinking for Episode I (even keep the age the same, just change the maturity), we'd have Episode III Anakin for 2, and then for Episode III we'd have Vader from the OT's maturity level.