This section of the Canon vs. Fanon series will focus on a fanon affectation which can be seriously irritating to many readers and might be enough to stop some of them from reading a story entirely. I can't speak for anyone other than myself, of course; but I've seen many other fans making the same complaint, so I don't think I'm alone in my feelings.
Brief recap: Canon is defined as anything directly shown during Stargate: The Movie or episodes of the show, with show superseding the movie when there's a contradiction. (Katherine has a photograph of Daniel aged three, or possibly four, shown with his foster parents in the movie vs. the canon from the show, which has Daniel losing his parents when he's eight.) Fanon is defined as widely-accepted concepts that appear in fanfic, but do have any real basis in canon.
My personal fanon opinion? My own reason for disliking "Danny" is my regular problem with overemphasized fanon in the first place: fanon, with its tendency to exaggerate aspects of the characters that are either marginalized or nonexistent on the show, results in writing characters that often have little to do with the actual characters I recognize from SG-1. So when I read about Jack and "Danny," I often feel like I'm reading about two characters that have nothing to do with Jack O'Neill and Daniel Jackson at all. And I think, in the final analysis, that the condescension in the name "Danny" is the aspect that annoys readers the most. Too many writers emphasize Daniel's "innocence," his civilian status, his comparatively poorer weapons training, and his greater need to be defended against enemies; add the younger-sounding "Danny" to this mix, and the implication is made – even if it isn't intended – that Daniel is of lesser status than the rest of SG-1, or at the very least has an uneven father/son relationship with Jack.
I know it can be tempting. I have a crossover fic that almost demands the use of "Spacemonkey" as an actual plot point. But I can't bring myself to use it, because it would be wrong.
"Anyway, I'm sorry, but that just happens to be how I feel about it. What do you think?"