I already gave my intro answer, thought I'd try to describe what wow'd me the most.
There was something about Daggerfall dungeon crawling that set the blood pounding in my temples. Unlike any other game, Daggerfall dungeons terrified me. The sounds of doors creaking, rats squeaking and zombies moaning helped my imagination run wild. The real world close in on me. The air became heavy with a musty moldy smell. The red brick walled hallways became dark dank slimy stone corridors where death lurked around every corner. Combat was an intense fight for survival as my mouse became the weapon in hand.
Good lord, it's an imp! Heart racing, breathing labored, exhaustion set in as I chopped, thrust and slashed left then right in a frantic effort to vanquish my foe. Thank the gods for the ebony dagger!
But it went beyond that. Right at the start, with the dice rolling in the cup, Daggerfall's character creation entranced me for days on end. Probably the best char-gen ever, with all its options I created dozens of characters before stepping foot out the Privateer's Hold.
All in 1st person, I was the center of the action not someone watching from above. But, I could see myself if I wanted by looking in a mirror (inventory menu). I ran like the wind and fought like Conan the Barbarian. I was Merlin with a wand, rode a horse like a jockey in the Kentucky Derby, climbed walls like Spiderman and ran screaming like a baby when I heard a ghost yell, Vengeance.
I remember, was it Sentinel, a long pool in a courtyard that lead to a castle. All that blue water in the desert. I couldn't resist and dove in to swim laps for an hour. Once deep inside a dungeon I stumbled across submerged ruins. Standing high on a ledge overlooking a flooded room below, I conquered my fear of the high dive and dove. Like in a dream I saw myself executing a graceful swan dive into the pool. Solid 10s. Don't count the 3.5 from the Orc judge, what do Orcs know about grace and beauty anyways.
The towns had life. Banks for money changing, stores for selling your loot or buying a new shirt. You could find odd jobs for hire ranging for simple rat extermination to rescuing wives and daughters being held for ransom, just get it done in time. There were temples to easy your pain and save your soul after spending the night with a strumpet in one of the seedier inns. Busty women strolled about the towns and chicks in chainmail passed as battle mages and castle guards. Daggerfall was definitely an advlt game, and I reveled in it.
Why, by chance if you might stumble upon lovely little nymphs and all sorts of other exotic wildlife. Ah yes, the nymphs. Met one once and I've been searching for her ever since. The deep dark nights in Skyrim do get cold and lonely.