If you have been wondering about the general organization of Field API in D8 and did not take time to work yched's existing D8 Field API sandbox, here is a simplified and cleaned-up version of the currently envisioned class and interface set.

Note that it is only here as a "thought-in-progress" step on 2012-08-19: actual implementation, when it is completely done, is likely to differ on some points, but this should at least provide some insights as to the general ideas. Thanks to yched for reviewing the initial version of this graph.