Form API: the next generation (FAPI TNG)

I'll just keep this short and simple. Longer stories can be found in my blog. See the links below.

I started working on a new form API because I hate the way the current API works sometimes and there are a lot of others who feel similarly. Key features so far:
- OOP
- More flexible validation
- No more hook_forms()
- No more base form bullshit

Biggest roadblock: elements are now classes. How do we make extending those as flexible and as clean as possible?

Resources:
- Form API: the next generation (introduction post)
- FAPI TNG: how and why it works
- Source code (Drupal 7 modules).