The new viewer doesn't use the full spectrum of HTML 5 features, to maintain compatibility with older browsers, but it would not be possible before HTML 5.
Using the HTML5 doctype lets you use HTML5 tags and custom data attributes and have a valid document. New HTML5 form fields, custom attributes, and markup elements are usable in IE6 mainly because it just doesn't really bother to explode when it encounters them. Form fields just show up as text boxes, custom data attributes are only used in JS anyway, and new structural elements are usable and styleable in IE6 just by adding JS that does a document.createElement().
HTML5 isn't something that just came around. It's been in the works by browser makers for quite a while, which is refreshing. Rather than it being a spec made up in a purely academic environment (XHTML 2), it's something that's made up of technologies that have already been used by one or more browser makers (and often, developers on real sites.)
Also, using the HTML5 doctype in IE6 causes IE6 to go into standards mode, which is just pure luck.
You can do a lot of good for users if you start using some of the HTML5 features right now, even if it's not apparent. If you use the type="email" for your forms when you ask for an email, the ipod and ipad will bring up the Email keyboard layout. That alone is kinda cool.
Anything else besides custom fonts?
Unless there's a subset of html5 features that I'm completely unaware of, I don't see how html5 brings anything useful to a text viewing app like scribd that wasn't already possible before...
text rotation, shadow or indenting, etc.
Lots of good 'print' looking stuff that was done with images or in flash before.
Now Scribd gets to do it in straight html making it easier to index as well (I know flash in indexable, but I'm pretty sure there is a preference to text).
True, but not convinced enough people care about custom fonts over rendering an image. It ends up being pretty much the same experience for them. (Actually custom fonts may well load slower for users, so it's a worse experience in some ways).
I do think scribd up to now has been pretty bad for the web, locking plain text documents and images up in their walled garden. Maybe they can change that, but what value can they actually add? What problem are they solving?
And of course, you know that users won't see any difference or care, despite never having seen, let alone used it. Just because you think the problem has been solved well enough, doesn't mean everyone does. After all, who needs a refrigerator when you have an ice box?