1. It says nothing about Io on the front page except "A programming language". That's cute or edgy or whatever you want it to be, but it's not a good way to market a new language and spread adoption. No user should have to click to find out what your product is. There is way too much clicking here in general.
2. Terrible use of space. The "navigation" (if you can call it that) on the front page runs past the fold on my screen. The text is huge. On other pages, the text is tiny and hard to read because it's white-on-black.
3. Layout completely changes for different screen sizes. I know this is popular, but when it can be avoided, it's better to avoid it. When people see the same layout on all screens (or as close as you can get), it makes usage of the site faster. For a site that has language docs on it, this is really important.