Here is the list from Microsoft. I will review a few key changes:

The biggest change, at least for me, is the removal of the Exchange Best Practices Anakyzer.

This tool has been a mainstay for countless consultants and Exchange Administrators for around 8 or 9 years. I found a post for the original release on the Exchange Team Blog here. Make sure to read that article as this was such a great tool for Exchange Administrators. In the chart it says the tool is retired. Note the tool was not in the toolbox for the Preview or RTM versions of Exchange Server 2013. I am curious as to what the replacement will be.
Another discontinued feature is the Spell Check in OWA. I know this has caused issues in the past either getting it to work or supporting it, however it was a useful feature provided by OWA.

I find the removal a bit harsh for end users. IE 9 does not have a built in spell checker and while their are third party products that can provide this functionality, this is an unacceptable solution for an enterprise product. There are other browsers with a built in spellchecker and I’ve read that IE 10 will have this; companies with previous versions of the browser will not have this feature. Hopefully Microsoft will reconsider this deficiency.
Access for Outlook 2003 users is now being officially discontinued.

I think this is good because of the benefits and features of Outlook 2007, 2010 and 2013 and the fact that Outlook 2003 is now almost 10 years old.
One last discontinues feature is the GUI version of AntiSpam and AntiSpyware configuration.

Now we are left with PowerShell only configuration. I will say that most of the companies I consult for do not use this feature or if they do it is used in conjunction with a third party solution. This may be a move by Microsoft to full deprecate this in the future.
Hope you enjoyed this brief review of some discontinued features in Exchange Server 2013.