This is strange because this is the opposite of what I would have wrote.
DK Tanking is now super easy (it was easy before as well as blood with WoTN buff), and while the rune management is a bit different, it shouldn't be too difficult once people get used to the priority system.
Mage dps is ridiculous, only after moonkins and maybe swd-trigger-happy spriests. I saw Adroit's logs earlier with him doing some nice numbers, maybe he can give you better advice, as mage is not my main.
DK tanking isn't any harder, it's just learning to use the new commands that is frustrating (you have a whole flow you are used to and it's like someone switched the notes on your piano). That and like I mentioned before, I find myself RP capped constantly (but apparently they are fixing RS to be back to just on GCD and not proc-based as on test servers). I end up DCing mobs right now.
I miss SCT. Parrot is just not as helpful thus-far.