I feel like EQ was "in between." It wasn't a pure sandbox, but there was a LOT more freedom in that it didn't feel on rails. Most of my time playing early EQ was at the mid levels doing PVP, crafting, dungeons and exploring. I didn't feel any pressure to really hit the level cap, because most of the stuff I could realistically get I could play the barter game and buy with enough effort (which in turn made those mid levels much more rewarding to play). Now, games are designed around fast-tracking to the cap. They're very much so more on rails. Things like Bind on Pickup, strict level caps, crafting caps, and so on definitely discourage lingering at the mid-game and just doing your own thing.
Eq wasn't a total sandbox, but I feel like there wasn't a huge amount of pressure to follow the beaten path. Everyone I played EQ with's fondest memories were of exploring the world, meeting people that we saw day after day, becoming part of the community, etc. You had the driven people who had to hit the cap asap, but even they tended to stop and go back to older dungeons and farm shit for alts or help out buddies. That's kind of gone now.
Another thing I think made EQ stand out was that zone levels and rare mobs were just really random. In games like wow, you have the 1-5 garden, the 5-10 area, the 10-20 zone, then a bunch of 30-40, 40-50 50-55, etc zones. EQ was just kind of all over the place. you had 50s in the same zone murdering guards as 30s killing goblins, you had 40s in zones with low 20s. Places like guk that ranged from 15 to 50, and so on. A wide level range, giving you some risk vs reward and incentive to really get out and explore, no matter what your level. I feel like moden MMOs is more like a straight ride. You ain't going back to older zones for any reason except obscure achievement hunts. Rare spawns dropping unique loot and mixed level zones seem like a thing of the past, and I think thats what made EQ feel more "sandboxy." Everyone was just spread all over the world, doing whatever the fuck, and low and high level players were in the same zones a lot, even if it was just a high level running to karnors through LOIO and dreadlands, it just felt more like a open ended world, and I didn't have any rails telling me where I needed to go. There were hotspots, yeah, but there were dozens of underused and interesting zones that rewarded exploration and just added to the scope for me.
I also feel like EQ just kind of rolled with things. I was always finding crazy new fun shit to do, like swarm kiting, faction warring, using dozens of interesting and unintended mechanics creatively. I feel like any time that happens in games now it gets instantly crushed, whereas EQ just kind of ignored it or even embraced it over time.
Finally, I think EQ felt a lot more sandboxy on the zeks as well, since a huge portion of the population was just fucking around and pvping ("half my fun is ruining yours") and not following any scripted goal. It was like a bully's playground, and it was fantastic for what it was. Like for awhile, de-leveling didn't reduce your skill caps, so you could run around using a tantors at level 20 (damage cap lifted) with the skill caps of a level 50. To some people that sounds like a horrible, broken mess, but to those of us who played there, all of those little quirks and odditys were just...fun. I feel like thats the kind of stuff missing from a lot of game's now. They're too sterile and just not fun.