OK I was joking about bringing up power levels lol. But the joke has some truth to it.
While I agree with you BF I do think that power levels DO matter in the case of Aizen. I think it matters because even if you're skilled and experienced you still wouldn't be able to get close enough to him unless your reiatsu was around the same level. So when I say power level I'm just talking about how much reiatsu a person has.
So in this case power levels matter very much and no matter skilled Ichigo is I don't think he has reiatsu close to Aizen or Yama yet. Unohana may have been surprised at Ichigo's reiatsu with his whole cloak being tattered and everything but I believe that was put in only because Kishi wanted to explain that part and he wanted a big tough character like Uno to *gasp* at Ichigo.
Or as everyone else likes to call it, make a WTF face