I think that because Christina has been out of the spotlight for so long, and it's been forever since she has sung, that because she came out and belted her ass off at a charity concert and made her face look like it was full of sadness, that critics were "blown away." It's true that at their primes, Mariah and Whitney were 10x the vocalists that Christina is/was, but she DOES have a powerful voice and right now she is one of the only technically great voices in world full of Ke$has and Britneys, and that's probably where critics are coming from.
While I do think Christina has a great voice, I also don't think she knows how to use it properly, nor do I think she ever will. I think it's too strong for her to handle, but in the current musical climate, she's technically the best voice out there.