This post is not about the anime and related works themselves, but the game as an element of the fictional world. Spoilers ahead.
Spoilers
There's the obvious: full immersion and "you die in real life", but I feel like there's more to the game that sets it apart.
First and foremost, progression in SAO was collective. For example, once a floor boss was beaten by anyone, a new floor would be unlocked for all players. This stands in contrast to existing MMOs, where players progress individually and interactions between them often feel optional. Of course, cooperative multiplayer games exist, but I can't recall a single one where this concept would be taken to a larger scale.
Secondly, since not all players needed to beat bosses, others could specialize. In most if not all RPGs there's a concept of classes and skill that impact how you beat the main story line and what side content you encounter, but in SAO you could make side aspects your main aspect, like the smiths we've seen in the series. I've found similar quality in multiplayer Factorio, a sandbox game of automation and tower-defense-like fighting, where oftentimes some players would focus on base building, while the others went out for combat.
It is mentioned in the series that quests are automatically created by the main system (what we would call procedural generation). While there were certainly hard-coded elements, infinite variety of procedural content is an interesting aspect, especially in times when almost all information about popular games can be looked up on numerous wikis. Starbound is a game that made heavy use of procedural planets, weapons, enemies and quests. For me similarities eventually became apparent and items lost the freshness, however the quality of being unable to perfectly min-max everything remained and I appreciated it.
Is there something else that you think SAO had, that existing games don't? Do you know other games that experiment with these concepts?
It's actually pretty unpopular in most games with hard end game content. Raiders want the best gear to come from the hardest content to make their efforts valuable and to feel rewarded.
SAO has a different nature to it though making a new zone and eventual escape the reward rather than focusing on gear. It also presents the idea of rare resources being better for making gear, the sort of thing that would immediately be farmed for drops so the gear can be sold off. But it also sells us on the idea that crafting takes some level of skill or at least that the results are unique enough based on skill and materials that they're usually aren't duplicates for rare crafts.
I must spend too much time around crafters lol
Personally, I prefer it because it brings more types of play into endgame, and encourages socialization. That's the kind of opinions I've seen as well. So long as raids give other rewards people want, it isn't a problem. I think being able to raid for materials is also very cool
Raiding for better gear to raid harder stuff is a cycle we've seen a lot. A broad feeling is that dropping materials to give to a crafter to get your gear slows and diminishes the reward and doesn't benefit things any. Honestly Wildstar's crafting was some of the best crafting I've seen. It was moderately skill based, made better materials important, allowed for alternate material components and allowed you to tweak the stats. So if someone wanted a specific stat distribution they could get it made that way, but a better crafter might make the stats better.
I'm still looking for a game that actually makes crafting skill-based and a good crafter better than an average one given the same gear and level or w/e. Until that happens crafters will always be an after thought.