One thing I absolutely DON'T want to see is a mix of styles. I'd rather see the lamest 1e/OD&D-style scrawls applied consistently than a jumble of styles cribbed from each incarnation of D&D.
Lack of clear, consistent art direction results in a product that's bland at best, visually incoherent at worst. Magic the Gathering's creative team figured this out many years ago, and they give every set clear, consistent visual cues that tie it together, even across multiple artists with significantly different personal styles. Eberron does a good job of executing a consistent style, which is one of the main reasons (along with the white, readable backgrounds and general use of white space) that books in that line are so much easier on the eyes than other 3e D&D books.