Engineering is ******* expensive as hell to level, since nobody wants anything you can make. Occasionally you can pawn off your materials on other
Mage is really fun. I've been having a blast on Arcane. It seems relatively simple from the outset, since you basically only cast one spell for like 80% of your damage, but there are a lot of things to pay attention to as far as mana management goes. Fire mage is also really fun, with DoT upkeep and whatnot. I haven't really found Frost to be enjoyable. It's alright in PvP, but I guess I'm just not very good since I find myself killed often enough that I don't feel it's very overpowered. Maybe DK is overpowered as well, since I have about the same relative success. Frost mage is just a lot easier to snipe killshots with. In fact, I've never actually had any issues against Frost mages as DK, since you have loads of abilities to get out of snares, avoid magic damage, and close distance.
I'm currently using both my DK specs as Unholy, with one talented for PvP and the other for PvE, which has worked out really well so far.