You asked why he was so weak compared to the other captains. That's basically because he's so young compared to most of them. And he hasn't been a Shinigami fot that long. Apparently he has extremely high potential. (minor spoiler incoming) Kyoraku (who has been a captain for about 1000 years or so) said that Hitsugaya will probably surpass him in another 100 years. So he is "weak" because he is still young compared to all the other captains.
QFT!