The most complete licensing comparison on Exchange 2010 is here:
And from the CAL chart there, we can see the detailed parts that are granted with an Enterprise CAL.
So let's detail these.
Advanced Activesync Policies
Within Organization Configuration, Client Access, Exchange ActiveSync Mailbox Policies, anything changes from the defaults on the Device, device Applications, or Other tab require an Enterprise CAL
You can see in these screenshots, that pretty much anywhere Enterprise CALs are being used there is an icon and a reminder.
If you have ever used an archiving product, you have probably used standard journaling. This is where every email written to a particular database is also copied to a single mailbox. Typically, then the 3rd part archive product picked up those emails and wrote them elsewhere. Premium journaling is under Organization Configuration, Hub Transport, Journal Rules. When you go to create a new journal rule, you see the same Enterprise CAL notification.
If you enable UM for a user, you need an Enterprise CAL.
There are two different Managed Folders.. Default and Custom. Default folders are your Calendar, Contacts, Inbox, Draft, Sent Items, Tasks, Etc. Custom is anything you want to create and deploy to your users outside of this. When you create a new Custom folder policy, you see the Enterprise CAL notification.
Integrated Archive mailbox is new for Exchange 2010. When you attempt to enable archive for a mailbox, you get the Enterprise CAL notification shown below.
Multi-mailbox search and legal hold
This is the Discovery Management role within the RBAC (Role Based Access Control) that can be controlled via the ECP (Exchange Control Panel) This one does NOT give an Enterprise CAL notification when you add a user to the role group.
IPC, Transport Decryption, etc
This is a multifaceted one, that is not highlighted as Enterprise CAL required when you configure it either. I will default to Technet for descriptions of each of these features, as they have them neatly collected here: