Survey of Confluence Features Missing From Confluence Hosted

Things that Confluence Hosted Won't Allow
Confluence Hosted has significant limitations

No clear information on how much data you are allowed. They claim unlimited spaces and pages, but 10GB limit on files, photos and videos
Can't add new themes
Can't add your own plugins
No personal spaces for individual users
Can not be firewalled for private inhouse use
No control over configuration and customization
No mail archiving
No user searching
Can't allow anonymous access, so hosted is probably useless for crreating a public wiki site.
Unknowns:
- They don't mention group permission lists. On Confluence, these are only available for editing by global system admins, so they may not be allowed in hosted version.
- API access is probably not allowed. API access would let you write scripts to manipulate your wiki pages.

Reasons to consider buying your own license for your own installation of Confluence...

Have your information on your own domain
Allows for public, anonymous, viewing of your information.
Better ownership of your data
- Making sure your data is not mixed in a database with other people's data
Automatic download of emails from an email account, and threading of those emails
Each user can have their own private or public wiki space to test ideas, or put more personal information
Creation of permission groups to control access to certain spaces or pages
Add new themes
Add new plugins
Allow anonymous access
Better control of comments on pages
CamelCase linking (traditional wiki style links)
Can enable trackbacks
You control when you upgrade your software
Access to source code
Can allow / disallow remote API access to your wiki.
Ability to customize various screens
Customize header and footer of your wiki.

Sources: