How many users are you planning on having?

Flexibility is important.  If you are planning on having a handful or hundreds of accounts, it is always good to have a provider that allows you to manage your own groups, Policies, and Roles in an easy manner.  If you do plan to have hundreds of accounts, a dedicated server is the way to go.  A guide on Picking a Shared or Dedicated Exchange Server can be found here.

The importance of helpful support.

Exchange manages important data, so support should be important.  Hours of support availability can be vital with different time zones and the popularity of outsourcing.  Be confidant that you will get the support you may or may not need if the event occurs.

Server uptime guarantee.

While shared servers are not a flawless option, they are often extremely stable.  Check out what the provider offers in terms of uptime.  If uptime is of critical importance, a dedicated server is probably the safe way to go.  If you are considering the dedicated route, make sure you contact the provider and understand the uptime guarantee and procedures clearly.

Is the provider validated and certified by Microsoft?

Microsoft Exchange is a very complicated service. You should feel confident that your provider is technically able to maintain your data. A strong understanding and relationship with Microsoft is vital. Exchange support is rated and certified by Microsoft. There are Certified Partners and Gold Certified Partners. A Gold Certified Partner is hard to attain and is a virtual expert in Microsoft Exchange.  Many people try to figure out In-house Exchange Solution vs. Hosted Exchange Solution.

How safe is your data?

How redundantly is the data stored? Since there is no fool-proof way to store data, redundancy is the only way prevent a nightmare data loss from occurring. Physical integrity and is as important as data integrity. Make sure the data is housed in a safe area too. Having your hosting providers servers located at the tornado or hurricane epicenter might not be the best idea.

The IMPORTANT small details that count.

How easy is it provision your new account/s? Does the provider have a easy solution to get you up and running? Do they have support features for the users? Some companies use a streamlined interfaces to get you on your way. Are there bandwidth restrictions? If they seem reasonable for your needs, its important to find out the rates for bandwidth overages. Is this provider dedicated to Microsoft Exchange? Do they offer a wide range of products? Although this may not be a negative, perhaps being spread too thin may cause support delays. It is just a good idea to know where they stand.

Remote Access and VPN accessability.

A virtual private network (VPN) is a private network that makes use of a public network (such as the Internet), while maintaining security and privacy through encryption and security procedures. The remote user can connect via an established VPN connection or use OWA. OWA is a slimmed-down version of the Outlook client that uses a web interface.
With Microsoft Hosted Exchange deployed in the enterprise remote users can easily get access to their information stored in Microsoft Exchange from their Outlook client. Dependant on how you plan to use your Exchange accounts, VPN compatibility can be important.