Communication protocols, database servers, software libraries, Adempiere support.
I worked as a software developer for Business Management Systems in Fairfax, VA for 39 years, where I designed protocol converters, packet drivers, database servers, and accounting algorithms. Since June 2013, I support Adempiere for Spanish House in Miami FL.
I have decades of EDI experience with ANSI, EDIFACT and other minor formats, and that is a major area needing improvement in Adempiere. There are major assumption errors in how ISA and GS identifiers are created, for instance.
On a less pressing note, I believe a cursor based API is a better fit than the PO system currently used in most of the Adempiere code. The two can coexist.