Still haven't finished this. I think I've only got the final mission left to do, or I'm pretty close as I think it's just a matter of talking to the Sun King and then heading to the nearby plateau with the antenna on it. I've downloaded the Frozen Wastes, which has removed the level cap (50), so I think I'll go do that before the final quest. I've missed a couple of the power cells, but I'm not sure I'm going to need the power armour as I'm already at level 50. Basically it boils down to whether I can be bothered to traipse around all the various locations — I know where they're supposed to be, but I can't remember which ones I have.
There are about 10 new skills to achieve, most of which seem to revolve around repairing over-ridden machines. Not sure how helpful that'll be, as it's not, as far as I understand, like you can actually direct them to aid you. Sure, you can fix your ride, but in large I've found it faster to get around on foot. You can collect things without having to dismount now, which would have been useful at the start of the game.
There are five, that I can see from the animal catalogue, new beasts, most of which start around level 30.
Some new weapons as well, though not sure how useful I'd find an energy firespitter.
Basically, if you haven't bought this yet, then I suggest getting the complete version with the DLC