To be honest -- a little of all three, but the third was definitely the straw the broke the camel's back, and in more than one way. I'm going to focus most heavily on this third point, because it is the largest contributor.

The label "unfamiliar territory" is a bit of a tree that splits into two major areas. The first, and the most obvious, is literal, physical terrain which I will focus on first as that is the one most people are familiar with. The physical terrain of Afghanistan is brutal and largely negates technological advantage in a way fighting in a flat desert or plain does not. The Hindu Kush is a rather unforgiving landscape that the Soviets had no real knowledge of. A good example of how unforgiving this terrain was (though not from the Soviet-Afghan War) would have to be the Kabul to Jalalabad Retreat of 1842, in which British General William Elphinstone marched (or perhaps more aptly, attempted to march) an army of 17,000 through the Hindu Kush from their cantonment outside Kabul to Jalalabad, where there was a British force. During the march, a great majority of the force died. Many sources say only one Englishman would return (Dr. William Byron, a surgeon), though others argue it was likely more alongside Sepoys. The point is though, the Brits took 95% casualties. It is arguably the greatest failure in British modern military history.

Another example of fear of terrain would be towards the end of the Soviet Afghan War, when the Soviets were moving northwards through Ahmad Shah Masood's Panjshir Valley. The Soviets threatened Masood, basically saying he should "stand down or we will kill you", to which Masood said something like "You can try". The fact that the Soviets had to tell Masood to stand down is extremely telling; they could not rely on their ability to gather intelligence to negate the Panjshiri Mujahideen. Intelligence gathering brings me to my next point: Human Terrain.

Human terrain was absolutely a larger contributor to Soviet (and indeed, American) failings in Afghanistan. There is a bit of a popular misconception that the Afghan Mujahideen were some sort of unified rebellion a la the Continental Army fighting for "Allah and Afghanistan". This was not the case. While of course, many were inspired by their God, and a minority by their country, to say they were unified is extremely misleading. Rather, Mujahideen were split into seven major parties. These major parties also seemed unified from the top looking down, but again -- not the case. Most of the lower level commanders split their tribal groups across party lines as to increase chances for survival, while also manipulating tribal ties in order to take out old rivals.

Now, where do the Soviets come in? Well, shocker, these low level commanders didn't just manipulate the parties. They got the Soviets and PDPA (Afghan Communists) as well. Simply put, they would tell the Soviets/PDPA something to the effect of "Yeah, Mohammad off in a Nad-e-Ali is a jihadi!". Now, whether he was or not is not much of a concern when you recognize that the Soviets also just made another jihadi that much stronger by knocking off his rival. Additionally, the lack of an ability to really build intelligence networks because of a lack of understanding of the local politics really killed any ability for the Soviets to have any foresight in the thinking of their enemy. This, in turn, resulted in their downfall to some extent.

Sources:

"An Intimate War: An Oral History of the Helmand Conflict" by Mike Martin

"The Return of a King" by William Dalrymple

"The Wars of Afghanistan" by Peter Tomsen