You may see some issues with Mail (particularly if you use Mail for Gmail or Outlook), Wi-Fi (occasionally issues reconnecting after sleep, or abnormally slow performance) or Bluetooth (occasional refusal to connect).
I have been using Yosemite for many months now and have only seen issues with the last one (occasional Bluetooth bullheadedness, a restart fixes it but annoying), but others have reported these issues. It's not apparently universal, however, so you may never notice anything.
There is a whole raft of new tricks your Mac can do if you have an iOS device around and a recent-model Mac (2012 or later), and a whole raft of general improvements. But as I find I have to remind people a LOT these days, x.1 or even x.2 is early days with a new major OS upgrade.