I got my first CRM Online trail with CRM2013 (it is available as you read), so back to 'school' to learn about new features, to find you way in the application. From user experience perspective it is much better, which includes new navigation method, new form lay-out.
Also new features as synchronous workflows, new database model are things to look for and start discovering.
One disadvantage is the fact that there is no return for current CRM Online user you will be migrated to the new CRM, so prepare for a change. Also the cost will increase now you will have three subscription levels 15, 30 and 65 U$ and of course you want the most expensive one. Of course its is according to many Microsoft presentation much cheaper then the major competitor Salesforce, just the messenger here :)