What happens when someone with an active account stops paying? I assume they're locked out; how long is their content kept around--how long do they have to reactivate before their content is deleted?
Filetypes: Do you expect to support .cbr/.cbz files?
The "unlimited books" books option... how unlimited is that? The company I work for regularly processes a few hundred mb of PDFs at a time. While those were never intended as ebooks (scanned legal documents, mostly), there are plenty of large PDFs that are ebooks--archive.org and googlebooks both have scanned PDFs. Between those and the ability to convert .cbz to .pdf, you could find yourselves storing dozens to hundreds of gb of scans for high-capacity users.
Or is there a cap on individual book size? (You could still find yourself flooded with large sets of 5mb PDFs, but that's less likely. However, if you don't accept scanned-image PDFs, which regularly run 12-40mb, that should be stated somewhere that people can see it before they sign up. Some epubs are 12mb, if they're heavy with pictures or code.)
Where's your TOS? Under what conditions can someone's account be cancelled, and what happens to their content & their pre-payments in that case?