If you are going to limit it to short posts, maybe the length of a short post is the length to limit it to, rather than emulating the 2xSMS of every twitter clone?
Other than that, it looks really clean and neat - I just dont think being 280 chars adds anything other than resulting in people posting with pagination as per X 1/3.
> Notes are limited to 280 characters. The constraint keeps them quick to write and read, making room for one thought at a time.
No thanks.