Currently, there a number of useful attributes about individual map service layers that are available directly via the ArcGIS Server REST API, but not exposed via the ArcGISDynamicMapServiceLayer class:
A couple of examples:
1) Expose the legend info. This can be obtained by requesting the "/legend" resource via the ArcGIS Server REST API.
2) Expose the "timeInfo"- It is not possible to determine which individual layers in a map service are "time-aware".