I came here to say almost exactly this (including the bit about loving the concept and pitch!).
The Twitter sign-up process was unexpected, and then randomly failed and took me to an error page.
I tried again and this time noticed that the check box that says 'agree to terms and privacy' doesn't link to the terms or privacy agreement, so I don't know what I'm agreeing to. This could get you into trouble in some jurisdictions.
Finally, the google signup worked with two clicks and didn't request a username/password - this is what the Twitter one should be like.
One more piece of feedback - I already knew how the product worked (it's obvious!), and I wanted to skip the walkthrough, but there wasn't a button for that. Add a 'skip' button to ease frustration :).