As currently stated, if a comment's never endorsed out of pending, the commenter will never be able to post again?
Edit: Deletion is currently possible for less than that (2 hours?). Will this be different for pending comments?
On a related note, the new system may prevent people from posting in the feature suggestion thread (https://news.ycombinator.com/item?id=363). Since it's already a special thread that never closes for comments perhaps comments posted there should not be made pending.
I suppose that the 24 hours is a parameter that is easy to change and tweak. I think it's a lot of time. What about 1 or 2 hours?
Another possible idea is to have up to two pending comments, so someone can contribute in two threads, go to work/eat/sleep, and read the replies. But this version is more difficult to implement and more difficult to explain.
Also some visiable indication on near the user/logoff part of the page indicating if can post or awaiting approval of a pend - say a "+" indicating can add a post and a "-" indicating you can't as have pending post.
At least, I don't get the impression that this update is intended to prevent such posts regardless of content (eg a reference to a related paper on some obscure topic).