And, of course, the crazy chicken-rider Dunestrikers are absolutely amazing. I'm just blown away by them. Their general look is mad, their details tie-in beautifully with the Knights, and their rules look very fun. I can't wait!
I'm a bit less keen on the stalk-tank, but maybe my mind will be changed by better pictures.
And the idea that this is just the beginning, that Codex: Skitarii might be followed by further codices and eventually a full AdMech codex (possibly including the Knights as well)? That's very exciting!
http://www.bolterandchainsword.com/topic/302401-rumours-mechanicus-skitarii-more-pictures/ is a great thread on Bolter & Chainsword with all the pictures and lots of rumours.