Cool. Inspired by the same reasons as you, I too was thinking of creating something like this but you've just beaten me to it. :-)
Now, my 0.02:
(There will be some duplication [read reconfirmation] of suggestions others have already made here and some new suggestions.)
1. Yes, don't force Twitter as the only sign-in option. (If you want, we can contribute a plug-n-play type e-mail authentication sub-system to save you the time and trouble - let me know.)
2. Create a separate (exclusive) page for each idea and allow people to post comments/replies.
3. Allow posters to (optionally) upload images/sketches to explain the idea better, faster. (Show such material on the exclusive page.)
4. Many idea posters would also be having a thread on the idea running at HN and/or other forums. Allow posters to (optionally) provide link/s to such URLs (as a separate field). This way, everybody can get an expanded view of all opinions about the idea.
-HTH