What they really look for is variety. Variety of mediums, styles, subject matter. Color work, perspective, portraiture. Try to include a little of everything to get noticed.

It really depends on the specific art school. Schools that stress “fine art” (like Maryland Institute College of Art in Baltimore, where my sister went) want to see that you can handle a variety of media: watercolor, pencil, marker, etc. and can take advantage of what those media can do in order to create a specific effect. Typically they also want to know you have the power of observation and can translate your observations to the sketchbook (specifically, the human body–hands, faces, etc). Schools that stress more commercial art–illustration, graphic design, etc.–want to see that you can follow instructions and produce something clean that communicates clearly.

Good drawings, with creative touch, you also have a style in drawing right? Show your style!