I was going thru Exchange 2013 recently , a bit late but as we say better late then never.
Microsoft has introduced some noticeable differences & we are back to days of exchange 2003 where we have frontend
& backend server concepts.(in some manner)
- Hub Transport server is gone now, feature has been distributed between CAS/Mailbox servers.
- ECP/Exchange management console is also gone, EAC is introduced (Exchange Admin center)
- Public folder console is also gone, It is now part of EAC.
- Edge Transport server is also not available currently but you can use 2007 & 2010 edge transport server with exchange 2013 as of now.
- Microsoft Exchange search engine has been replace with Fast Search in Exchange 2013.
- Built-in DLP , for protecting users from sending personal information such as credit cards.
- Outlook Webapp has been greatly improved for phablet/tablets.
- Offline outlook web access (Very Cool feature)
There are many more changes but these are the ones that I have found to be very different.