What made PSO really special to me? The right question would be "What didn't?". But let's start:
- Star Wars made interesting:
I'm not a fan of Star Wars, but I like some ot its elements, and I loved the way PSO mixed the elements of Star Wars with new stuff that replaced or improved upon the things I didn't like from that franchise. I'm a huge fan of the whole Phantasy Star franchise, quality of some games aside.
- Discovery of Phantasy Star:
I know the series existed, but PSO was the first game I played. I know that, strictly speaking, it was a different game series, but I started playing the old games thanks to this, and the first Phantasy Star is now one of my favourite videogames of all times. Seriously.
- Diablo-esque gameplay:
Simple, no-brainer hack-n-slash action with no real enemy AI and a plot almost irrelevant to enjoy the game was good shit. I liked the story, specially when they expanded it in Blue Burst, but I didn't need it to have fun, and the way enemies worked on PSU and beyond made me enjoy those games a lot less.
- Horde crusher:
Someone pointed something like this before, but I'll do my version: melee weapons that hit up to five enemies in just one swing, strong attacks that made enemies flinch, attack chains to combine at your convenience, and even units that raised the speed of attacks made me feel like a superhero. I could keept huge hordes of enemies at bay by hitting a lot of enemies with a combo, retiring a bit, and starting a new chain with the enemies that didn't flich on the previous round. Throwing so many enemies at me in a single room also satisfied my bloodthirst. Playing as a Ranger or a Force wasn't as fun as being a Hunter, but I played a HUnewearl just in case I needed to zonde those canadines.
- Decorative Mags:
I used them to define my character's look, and I was very careful at raising them so I didn't lose too many points in stats I didn't want. My usual choices were wings-shaped mags and shoulder companions.
- Setting:
I think they were really smart with the plot: they fucked the on-land colony, but the ship had came to far to come back, so they had a coherent hub map stuck in orbit. Previous settlers were the ones with the big army, so you can find loot manufactured by your people already abandoned on the new planet. Yours is a ship with a metropolis inside, so you have plenty of unknown people to make them become player characters. Since bounty hunters exist, you can have a military character operating independently as a merc but still recruited by the government, so they have a way to assign you missions while still letting you explore on your own. There's probably more, but that's enough to get my point.
- Level/environment design:
While it could sound stupid with the info I know now, back in the day I thought it was obvious that game's levels were designed by Sonic Team; I mean, they looked like the kind of places you would see Sonic running through had the game be a platformer and not an action RPG. IIRC, only some elements from Ruins 3 and of course the Dark Falz arena were unlike Sonic games. I've had a similar feel with most of the levels in modern PS games, if not all of them.
I maybe forgetting something, but that's all I can recall now. I used to call this game "Phantasy Star Offline" because I didn't play online until I tried a Blue Burst private server a while before PS Zero came out, and never online again until I entered PSO2 for the first time when episode III was already happening. I'm not very fond of online multiplayer in any game, specially with random people, so I suppose I'm really different from other posters from here. Not sure if it's good or bad, considering I enjoyed this game that much just by playing it alone.