Uhm.....no he isn't actually. He's just meant to look uber powerful when most of the captains could beat him in a heart beat. Remember, just because he's a vizard doesn't mean he's all powerful. Unohana is one of the most powerful captains in Seireitei, dwarfed only by Shunsui, Ukitake and Yamamoto. Though it has been implied by Kubo that she is just as powerful as either Ukitake or Shunsui. In otherwords, she could pwn Ulqui in a much more timely fashion than Ichigo.
She is far more powerful that Ichigo, in fact, several captains are. Soi Fon was only beaten by Yoruichi and we know that Ichigo can't beat Yoruichi due to the simple fact that she is simply too fast. To list off a few shinigami more powerful than Ichigo here we go.....
Shunsui
Ukitake
Unohana
Soi Fon
Yoruichi
Kenpachi Zaraki (Kenpachi pwned Nniotra when Ichigo couldn't, we know this is true)
Kisuke Urahara (potentially due mainly to his greater intelligence and ability to forsee the enemy's movements)
Well that's just the basic list, not even including the other Vaizards. Remember folks, just because Ichigo is the main character doesn't make him the most powerful guy in the series.
