Mapnik Expressions enable data-driven behavior in Mapnik. They are constructed from strings, can hold any kind of type of data, and are evaluated at render time. They are used by symbolizers like the TextSymbolizer to dynamically construct text labels for display and they are also the basis for how Mapnik Filters work.
Recently, in Mapnik 2.1, expressions became enabled in the transform parser used to apply affine transformations to image and svg symbols. So, now you can not only, for example, rotate symbols with a transform like transform="rotate(45)" but the rotation can be data driven like transform="rotate([field])"
For more info on the original design see: http://mapnik.org/news/2009/12/08/future_mapnik2/
in the feature,I think the color attribute such as the stroke of LineSymbolizer, the fill of PolygonSymbolizer, the fill of TextSymbolizer and so on, should also support Expression, this is more useful.