One addition which is present in all of iWork's applications isn't exactly built-in. Instead, it comes as an ability to interface with Apple's answer to Google Docs and Microsoft Office Live, iWork.com - currently in beta.
Documents from Pages, Keynote and Numbers can be uploaded to iWork.com either for access by their uploaded from another location, or to facilitate others making a contribution to said files. Viewers can either add comments online, or download the files to make changes themselves.
Format support is the same as all other iWork sharing offers, with iWork, Office and PDF documents all sharable. There's a simple but effective interface showing which documents are shared and with whom, from which usage rights can be changed or revoked.
As beta software it's hard to be that evaluative of iWork.com, but the early signs are good.