Why don't you allow browsing without registration? I am sure you will miss out on a lot of traffic and future sign-ups because the initial experience is opaque.
 Don't look here for a link from me, I'd rather not encourage them by sending traffic their way.
Came here to write a comment, then decided to go back to the site. It accepts firstname.lastname@example.org as a valid email. Typed throwaway password.
Tip for others: Just register with fake details in order to see what the site is. There is no email verification step. If it takes your fancy just re-register again properly.
Edit: I realise I sound a bit negative, so I want to say that everything I didn't mention was a positive. The site looks beautiful, what content is there is fantastic and you should definitely keep it up.
I wanted to say yes, this. Allow viewing of everything but to participate, you have to register.
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.
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.
So first year of uni tops, where most of calculus is still high school.
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.
I would say though that I think your articles and lectures could do with a bit more organisation. In both cases, the user would benefit from posts having tags denoting subject content, and from a search facility so that I can find things that specifically interest me. If you have five hundred videos about mathematics, finding one on Analytic Topology, say, by scrolling through them would be a bit painful.
I also think it would be good if the lectures had some way of tying subsequent videos together - it seems a tiny bit messy to have 'Cosmology - Lecture 1', and 'Cosmology - Lecture 2' as separate entries.
I'm also not certain that you want to limit lectures to being videos. A well-written explanation of a broad subject is sometimes better than a video that I can't search or skim through - and I think it serves a different purpose from the articles. It's also easier for your community to add written content than videos.
Hope that was useful to you - but obviously take it all with a pinch of salt. You know your strategy better than some guy off the internet who's only spent ten minutes on your site :)
A suggestion: Redis has a very cool documentation system, where the documentation is "live" - for example, http://redis.io/commands/set (scroll down to the example, you can type in the box). It'd be really neat to have something like that integrated with the code side of things.
Saw registration screen. Closed window.
of course... in the time I typed this I could have just signed up. :P
I would expect your site logo in the top left to take me to your home page, especially as it appears clickable, but it doesn't. It puts a # at the end of the url and that's it.
Heads up: I left the site, and it didn't remember me when I came back. I forgot my random password and requested a password reset. Got a confirmation that an e-mail had been sent, but that hasn't arrived after a good 30 minutes so far.