Eh, I wouldn't call EQ1 a sandbox in most respects. It had defined classes (no maleability, no "you can do whatever you want aspect) as well as skills/spells that were specific to those classes. There was 100% a real progression of content and dungeons from first level onward, with major bottleneck congestion appearing at certain intervals. Yeah you could theoretically go anywhere shit was able to be killed and kill it for xp, but if you wanted good xp/loot (the point of the game regarding progression) then you went to specific places. In WoW, for example, you could do precisely the same thing minus killing guards. It really was the same experience as early WoW except it was less sandboxy in the class design area. You couldn't "respec" in EQ, you were stuck with whatever class you chose. In WoW, a warrior could be a tank or usele...er, dps, while warriors in EQ up through Luclin at least weren't exactly DPS machines.
My assumption with the term sandbox is that you are capable of doing whatever even if it doesn't work or is inefficient. Ie, you make the game, the game doesn't force you to do anything. Technically, no game forces you to do quests/go to hubs/go to KC/whatever. It's usually an extremely lopsided choice in efficiency and reward vs. substantially less in most cases. Thus, the logical choice is to follow the path of best results, which typically was Befallen/Paw/Lguk/SolB/KC/Sebilis etc etc. The progression really was almost identical to WoW, you just didn't have the path so clearly laid out. And their typically wasn't options for the most part between areas when it came to quality of drops and rate of xp for progression unless you -really- consider Sol B's drops on par with Lguk's.
I'm all for removing question marks above npc heads and making maps more like a map (which didn't matter that much in EQ anyway because there were well done map packs within days of it being implemented if I remember correctly and most players DL'd those at the first chance) where you can draw your own but the game doesn't do it explicitly for you. I'm also for more class diversity but with the ability to mix and match a bit more instead of just being "You are an SK, you get these abilities at these levels and these spells at these other levels. Revel in your uniqueness."
Less directions could be good, but areas should still flow into one another sensibly when it comes to content difficulty and flavor. But yeah, as to what precisely a "sandbox" is will depend on the person telling you the definition. To some people, sandbox means they can grief freely and some mythical societal pariah status is all that keeps them from going postal at a moment's notice. The ability to deny people content or destroy other people's things or kill them at the click of a button makes the game a sandbox to some. Some people assume it means Minecraft with classes. Others (myself) equate that, in PVE terms, to being able to enjoy content at your own pace and without restriction in most cases with ingame systems to support progression in a variety of areas so that being a farmer is just as possible as being a dragon slaying knight of evil. Having quests doesn't negate a sandbox.
Of course, the one thing I believe most people agree on is that a sandbox game doesn't have multiple hard factions. Everything is soft. And by hard, I mean you cannot change it no matter what you do short of out of game services such as faction transfers et al.