The Ethercaste Knight proves that not every card in Alara Reborn is a gold card. He still has multiple colors in his casting cost, but he’s an artifact creature. Also, knights riding lions? Nice.
I’m honestly not sure how the designers justified the Ethercaste Knight being an artifact creature. The flavor text seems to suggest that his etherium armor is the source of his artifactness, but the knight himself is a human. Wouldn’t it make more sense to have a white/blue equipment artifact called Etherium Armor that granted the wearer a similar Exalted effect? [Edit: Ok, now I get it - his arm shows that he's fused etherium to himself. Cool.]
Ah well, whether or not it makes sense, this card is an excellent common for those who like to build Exalted decks. I’ve run into a few of them, and I don’t like playing against them at all. With a bunch of Exalted powers in play, you pick one creature to be your “champion” and attack by itself each turn. It gains a bunch of abilities (life gain the worst of all, in my opinion) and is really tough to block or kill.
The Ethercaste Knight helps in a simple way. He only costs two mana to play, is hard to kill with a toughness of 3, and boosts the Exalted champion with a +1/+1. Since the knight will be staying untapped while your champion attacks, he’s there to block when necessary, and a 3-toughness blocker is a nice thing to have around.
All in all, this is a good common, a very meat and potatoes card for Exalted decks. If your deck isn’t built around the Exalted mechanic, however, he is somewhat less useful. In limited formats, he provides a decent blocker, but the Exalted ability is likely to come into play only in the early game. I wouldn’t draft one early, but if a few were passed to me and I was playing blue/white, I’d certainly include him in the deck.