iCloud Document Library as replacement for Dropbox?

No, as iCloud is supposed to be for personal use only. To, in this case, share documents between all your devices.

You could always share your password and let someone else log in and download the file, but that will be insecure, of course. Extracted from Apple's iCloud page:

Just sign in to icloud.com/iwork in any modern web browser, and all your iWork for iOS documents will be there — complete with your most recent edits. Click a document to download it in iWork ’09, Microsoft Office, or PDF format

Edit: Documents eligible for sharing with this new feature, are those created or edited with compatible Apps. Therefore, I don't think you'll be able to share any kind of files through iCloud.

iCloud is already seamlessly integrated with the Apple iWork apps for your iOS devices — Pages, Keynote, and Numbers. And we’ve given developers the tools to make their apps work with iCloud, too.