I think crackfic indicates that the story is intended to make the reader sporfle. Centaur stories are a great example. They ought to be crack, the premise is crack, but in James' hands, they're not (though she's perfectly capable of writing crack, it's just more likely to have penguins than centaurs). The premise is "these people are centaurs" and it's treated as a serious jumping off point for drama, plot, and character. Crack is more "look! I can make Ronon into the Hamburglar! Whee!"

What's always weird is when crackfic becomes non-crack. Sometimes it works (the entire Sith Academy was crack, but it had non-crack entries near the end, and the entire Hell Series spinoff was non-crack (drama and character development) spun off from crack (Obi-Wan and Darth Maul live in the same apartment building). Other times, it doesn't work as smoothly, and badifc tropes that were games in the crack get taken seriously in the sequels to the detriment of the fic.

On the other hand, it's a nice safety net for risk taking and writing guilty pleasure fic (When in Haldoria comes to mind).