I'm a classroom teacher, in mathematics and computer science. I can say, with certainty, that no teacher at my school is threatened by Khan Academy. That's a straw man. We became teachers because we want students to learn, and any resource that helps them do so is wonderful.
If we do criticize Khan Academy, it's because we're less than thrilled with the implementation and instruction, not the site's very existence.
I've spoken with more than a few teachers who are abjectly opposed to KA content. While my own observation is that of a concerned parent (small sample size applies), I would suggest that there are some teachers who exhibit the behavior of having their approaches to teaching second-guessed (hence: threatened.) By a similar measure about "straw men", I would double-check any assumptions about teachers and their desires for students to simply learn. I've personally dealt with those in the teaching profession who resist the idea that there's more than one way to learn something.
As for KA criticism, I've yet to hear suggestions that would yield better results. I get that implementation/instruction goes against the grain of traditional teaching methods. But hey, maybe that's the point? I'm in no position to assess that, but I am in position to evaluate results.
My kids are proof positive that using KA works for them. I'm an engineer and excelled at math, and they understand concepts better with KA material than without, so I have some perspective as to whether their learning is what I consider well-grounded.
The implementation and instruction at Khan really doesn't go against traditional teaching methods. They are traditional teaching methods, transposed online. Learn via a one-size-fits-all lecture, practice via rote. It's a traditional method, mind you, because it's worked fro hundreds of years.
I'm not surprised it works for a number of students, and I'm glad it works. It's better than most alternatives out there, paid or free, which is damning with faint praise.
I'll be the first to admit: I can't match it, or beat it, because it's a herculean effort to do so. One of my best one-day lessons has probably taken me 15 hours of work outside the classroom to write, over the course of 5 years, and has seen a half-dozen revisions. I'm still not in love with it. If I complain, it's not because I want KA to go away, it's because I want it to improve.
During my tenure there, we won the Broad Prize for Urban Education, and were specifically noted in the award for raising our 4th and 8th grade mathematics scores more dramatically than any other city in the country.