Sorry to bring up an old thread, but this caught my attention and I had to reply. Reliability, clarity, accuracy - that's all good, and you should certainly hope for all that in a good optic, but I think the question is HOW do you determine all this? There's a couple things you should be looking for in any optic, most of which have some wiggle room depending on what you're looking to pay.
Always look for waterproof and fogproof. There isn't an exception to this, even if you don't hunt in inclement weather. Optics that aren't fogproof aren't nitrogen purged and retain some humidity in the air that is inside the scope. Rapid temperature changes (like going from a cold winter day to a warm hunting cabin) can fog up the interior surfaces of lenses in non-purged optics and may take hours (days?) to clear up. This could be a huge problem if you're paying for a hunt and you didn't bring your backup riflescopes.
That also brings up an interesting point about waterproofing. Binoculars, spotting scopes, and riflescopes are only technically waterproof if they are nitrogen purged. Even then, only the lens bodies are sealed, and any moving parts are technically still vulnerable to moisture damage. But unless you're hunting underwater, it's not much of a problem with proper care. Bottom line: Nitrogen purged or it's crap.
As far as lens coatings, there are several different types, the main ones being non-coated, multi-coated, and fully multi-coated. Non means no lens coatings whatsoever, and the least protection from dirt and scratches. Multi-coated means the exterior surfaces are protected, and fully multi-coated means every lens surface (both interior and exterior lenses) have several coatings. Each manufacturer has a different catchy name for the type of coatings they use, but generally speaking, fully multi-coated optics have the best light transmission and abrasion protection available.
When it comes to bodies, good optics will have some inherent durability against field use, which means rubber armor on binoculars, bodies made of lightweight materials like aluminum and magnesium. And one piece construction on riflescopes, usually aluminum. One piece scopes are going to be much more rugged than multiple piece, and are way more likely to retain a zero after you drop it from your treestand. The same rules about nitrogen purging, waterproofing, and lens coatings apply.
Aside from all this, what it really comes down to is personal preference. Weight, size, magnification, effective range, color, turrent types, lens caps, name on the box... All that is superficial stuff that is going to vary greatly depending on who you ask.
Hope that helps.