If you are using the Silicon Labs CP2102 USB-Serial bridge the behaviour of the RTS line changes.
Previously this line changed state when the port was opened by the host, now under the new OS it does nothing.
This is important because it can be used as an auto reset on the Arduino embedded board to synchronise the boot loader with the downloading software. No other pin changes on opening the port so while you can still communicate you have to manually reset the board for downloading.
I have contacted the Silicon Labs support team and they have promised an update in 3-4 weeks.
Note this does not affect the normal Arduino boards as they use an FTDI chip, this is only an issue if you are using the Silicon Labs chip.