My two highest priorities are progressive enhancement and inline editing. I've found progressive enhancement (DataTables) and inline editing (jqGrid), but not both. Support for jQuery UI themes would be nice, but is a lower priority.
UPDATE: Here's an example of what I'm imagining the solution would resemble:
<table summary="A table full of example tabular data">
<caption>My Table to Progressively Enhance</caption>
<th id="colA">Column A</th>
<th id="colB">Column B</th>
… insert jquery datatable stuff here …
With the latest release of jqGrid, we now get
tableToGrid, which solves the grid-from-markup problem quite nicely.