Zimbra is based on requests (how do features get into zimbra?) to that end I see in another of your threads that you've seen the v4.5 demo, but checkout the v5RC1 demo for some new RFE's that got implemented like tasks, briefcase, and aggregated external imap accounts (external pop is already in 4.5). IM is well on it's way & voicemail like some providers are doing are being worked on but you can see some early functionality. (another IM teaser)
What most of the hosting partners have done is about 3-4 levels of features & storage. At their highest/business levels they give you access to the delegated admin which can only control accounts/aliases/resources/distribution on one domain (think the top right section if you've seen the admin console). Thus they don't have to get a phone calls/tickets 'please add this account/change this setting' and they don't have to spend hours making a custom portal like raj has nicely done for his FOSS install.
Compare ZCS Features
A brief rundown:
Hot Backups - I can't begin to express how useful those are! - The FOSS edition has cold or 'warm' backups, some take snapshots of an xen, or fork mails, etc, etc; but their ability to quickly restore just one account exactly how it was at a point in time waayy lengthier.
Hierarchical Storage Management (HSM) -easily offload data to another storage solutions automatically based on date. ie: Automatically move the messages after say 30 days from your fast SCSI (/more expensive solution & therefore you tend to have less of it. To a SAN of SATA's or something. If you we're to have some performance bottlenecks with this cheaper storage arrary, the end-users don't notice any lag because when they search they see the recent results first anyways.
Clustering/High Availability on RHEL/RHCS (there's FOSS methods using heartbeat/DRBD)
Attachment Indexing -search inside that pdf or word document
Attachment Rendering/Conversion to HTML
Outlook (ZCO) & Apple iSync connectors
Zimbra MobileSync - OTA push & carry your mail/contacts/cal on you phone
(the MobileEdition web-client you see in the demo be free to both versions; it was originally called iZimbra but any browser works)
Archiving & Discovery (ZAD)
Delegated Admins -as previously described above