Is there anything that CAN'T be replaced by Style Sheets?

Quite a bit actually. Style sheets only specify information that controls display and rendering information. Virtual style elements that convey the NATURE of the content can not be replaced by style sheets, and hyperlinking and multimedia object insertion is not a part of style sheet functionality at all (although controlling how those objects appear IS part of style sheets functionality.) The CSS1 specification has gone out of its way to absorb ALL of the HTML functionality used in controlling display and layout characteristics. For more information on the possible properties in CSS, see the Index DOT Css Property Index.
Rule of Thumb: if an HTML element or attribute gives cues as to how its contents should be displayed, then some or all of its functionality has been absorbed by style sheets.