Also, no serious Wikipedia article about math or physics is thought of as an introduction. Sometimes its the other way around, its written for experts and goes into surface or line integrals, or sometimes even tensor notation, when an introduction would not.
I think this website is trying to reduce friction for user interaction and aggregating it, and also tries to curate material into different levels of difficulty. Both things merit asking for a login, for social its obvious why, and to curate for difficulty its helpful to use the feedback from users as a signal of an innadecuate label.
I would think a site where people can ask anything, ala stackoverflow, would yield more useful results for many physics/math/cs students.
That being said, I wouldn't take that as a reason to not pursue this.
So first year of uni tops, where most of calculus is still high school.
Similarly, if I go into the physics topics, they are all <= intro courses, at best. Optics? Doesn't talk about fourier optics, lasers, etc. E&M? Doesn't seem to mention Maxwell's Equations anywhere... enough said. Quantum mech? Nothing. Statistical mech? Nothing.
Okay, how about math? Take a look at the differential equations stuff. No higher order, no series methods, no numerical methods, no coupled systems, no non-linear. Probability? No markov chains, MCMC methods, or anything except basic RVs and statistics (which is not the same thing as probability). Same for linear algebra: no fitting methods, no matrix decomposition, no graph theory.
I could go on. The point is that they cover the most basic elements of each subject, and they miss a hell of a lot of important subjects. That's okay, but don't claim that they're anywhere close to being able to educate you at a university level.
Besides, just because khan does similar things does not reduce the value potential this has.
There is a lot more of a social SO type feel with Function Space whereby a user can ask questions and receive answers in an open forum. That is more hidden on khan.
For those of you complaining about the login. Give it a shot, there is a fairly well thought out site behind that login and the dev has expressed his intent to make things more open. With a site like this being able to ask and comment is kinda key to it working anyways.
Use your spam address on the site until you feel it's worthy of your "personal" one. But do check it out.
Free and open access to knowledge and knowledgeable people needs to be done in as many places and formats as possible. I hope this goes far.
O.