If you start looking around at Exchange Hosting offerings, everyone takes prides in their great features, industry experience, etc... In this post, I expect you to understand what you should be expecting as a baseline and at minimum in the bundle.
Lots of free goodies are now "standard" when they sell hosted mailboxes such as Spam/anti-virus protection, license for Activesync/outlook, 30-day money back guarantee, instant activation, flexible contracts, etc. Hosters should also provide full backups, shared calendar/contacts and outlook anywhere. These are somewhat necessary features to start with.
Come to the pricing - hosters will have a few plans according to single user or multi-user scenario and the price is fixed based on mailbox capacity. Usually it starts from 200Mb to few GBs. A slight difference if its a multi user plan, as the charge is based on a combination of the no. of users plus the required total mailbox capacity for these users (for example, if you take a 5 user plan with the storage of 2.5GB, then each user will have a 500 MB mailbox capacity).
How about some other value additions?
Nowadays, having a mobile device is not a value addition. It has become part of our daily usage. That brings me to services like Blackberry and Windows Mobile Devices. Both offers push mail, device security, flexible device administration etc.. Hence its users choose one of them.
Firewall - though the exchange servers are shared and located at the service provider, users will still need to bother unless hosters protect with an appropriate firewall. Make sure they have this implemented industry standard equipment.
Setup fees & customer support - a one time setup fee is common across all providers. Since mail is a mission critical application, don't forget the fact that customer support is important and 24/7 support by different means (phone, email, live chat, remote admin etc) is essential.
Last but not the least, get a written paper about uptime (most of them offer 99% uptime, the more 9s the better the uptime).