As others have said, quality. I think $799 is too much for the HD model, but closer to $600 it's a really nice scope for the money. I own the 10x HD (as well as almost their entire lineup now... 5-20x, 3-9x, 1-6x, 1-4x, fixed 6x non-HD) and it is a basic scope with very good glass, nice turrets, and a HUGE adjustment range for a 30mm tube (I believe it's somewhere around 36 mils or close to 130 MOA of elevation, if I remember correctly). It has typical made in Japan quality and should be rugged. Nothing fancy. I personally like the mil-quad reticle a lot, but some of that's just a personal preference and if you don't, obviously it's not the scope for you.
I've owned several Vortex Viper PST models and I would not put them in the same class as the HD model SS's - the 4-16x50 and 2.5-10x44 models are FAR behind the HD SS's in optical clarity (however, the 2.5-10x32 FFP model is not far behind the 10x HD), and all of them simply do not feel like they have the same construction quality or ruggedness. Of course they do offer a lot of features for the price, including illumination, if you need it and are willing to sacrifice other aspects for it. A much better comparison IMHO is the Bushnell Elite Tactical line, which in the $700-900 range gives you very good glass and construction quality.
The $299 SS scopes are a very good value. The $799 list 10x HD is a nicer scope. Not necessarily 267% nicer, but nicer.