The content or features need to be the main draw to your site.
I hate to see a novel approach like this poo-pooed because it's not a rubber stamp copy of big-web design principles, and without justification, no less.
The fonts changes are a little funky, and that's about the only criticism I have. It's easy to follow and usable. I really like it.
Some changes over time can improve clarity, but the initial friction is costly.
Ignoring stuff that others have already pointed out, there's no way to know which fields are required before you submit. Not a big thing in the grand scheme. But it's kind of a no-brainer, always recommended usability thing.
As for ignoring web standards have you looked at the markup? It's compliant HTML5: http://adactio.com/journal/1524/
I like the whimsicality of this form, but...
Do you think there's a chance that anyone, ever, will see this form before having successfully registered at more traditional forms dozens (if not hundreds) of times?
'Usable to someone who's never used another website' is not the most practical standard of usability. "Users spend most of their time on other sites," after all.
Huffduff may still for reasons of personality prefer this form, but I don't think it can be said to win on usability considerations.
Hi, my name is _____ and I'm a _______ who made $___,_____ last year. I already paid $___,____ through my employer and I have ___ children and will be taking _____, ______, _____, _____, and _____ as deductions. I had capital gains of $__,____. Also, I would like to donate ___ dollars to farmers and ____ dollars to political campaigns, and I am aware that any donations do not affect my tax obligations at all. This was filled out on the ___ day of the ___ month in the year of our _____, 20__.