No responses so far? OK, I'll give it a go.
To me, sparkling fill is entries that are (a) lively, (b) rarely used, and (c) have some Scrabbly letters. Lively is, of course, subjective, but to me means entries that are both interesting and at least a little surprising. A great example is DJJAZZYJEFF (at least if you were into 80's pop rap music): lots of Scrabbly letters and unexpected combinations. It also means avoiding contrived words like RESEATERS (ugh!).
Sparkling fill is, alas, like sparkling wine: it loses its fizz pretty quickly. So one shouldn't expect more than a couple of sparkling entries in your average themed puzzle (usually the longer nontheme entries), and maybe half a dozen or so in even a good themeless...but more likely 2 or 3 really sparkly ones, and a fair smattering of nice but less flashy ones.
Hope that helps.