Absolutely nothing. From my impressions as a long-time member of head-fi, this is a fairly popular company known to provide great bang for the buck, hence these kinds of reactions.
Audio-GD products measure very poorly but if you can put that out of your mind and not thinking about it, or care about it, they offer some products with nice feature set at interesting price points.
I get why Audio-GD is more popular on Head-Fi as Head-Fi is all about the subjective impressions while ASR is all about the objective part.
Subjectively Audio-GD is just fine. To be honest I can not for the life of me hear a difference between my Topping D30Pro and the Audio-GD unit I have during regular listing. I might notice (or think to notice due sighted bias) some difference(s) during direct A/B testing but that is not a realistic day to day use scenario for a DAC. This while the SINAD score of the Audio-GD is probably only half, if not even less, than the D30Pro. So to that degree I can follow what is going on at Head-Fi and why people feel that way about Audio-GD.
Where I personally do draw the line and really can not agree with on Head-Fi is post where people talk about things like "upgrading" their stock ACSS cable to new >800 dollar per cable and claiming to hear a
noticeable difference. But wait you'll not notice the difference immediately the cables need to burn in for 350 hours first. Not 100 hours, not 300 hours, not 400 hours, no 350 hours... where do they get that number from in the first place..??
Maybe it is super bad of me but I can't take the opinion of how something sounds serious from someone who believes in that snake oil.
I'm not that active any more on Head-Fi as for my personal taste people there cross the boarder between subjective impressions and pure snake oil fantasies too much.