I'm not even sure what it means to say that DAoC was linear. The story and lore didn't matter much in that game. The world was a big grab-bag of quests, with the exception of a few different "epic" chains here and there, and basically you quested and killed shit to get to 50 and get out in the war as soon as possible. Some individual quest lines were very linear (see 'chains' above), but other than that it was all about leveling up as fast as possible. Our guild used to run leveling nights where we'd go on runs with fifteen or twenty new toons, and a bunch of 50 PBAOE Spritmasters and do some place with loads of purples. You could get ten levels a night, easily.
Correct...however, items had level restrictions, areas were clearly tailored to specific levels and if you didn't have a level 50 toon to PL you, you would travel through the world in a predefined path using the same items as everyone else at that level.
THat's what it means to be linear.
While DAoC didn't take it to such an extreme as Warhammer Online did, it was still linear. Hence why I called DAoC the most fun game since the trend to linear games started.
It's inarguable that most MMOs to come out have been extremely linear. WoW is, LOTRO is, WAR is, SWTOR is...they all are. None of them are at all sandbox-like. Even TSW and GW2, both of which were proclaimed to be fluid and open, are linear.
A lot of it (ok, the majority of it) has to do with having a leveling system. When you have strict levels, rather than organic skill and stat progressions (like UO had), then you restrict yourself to having a stepped approach to progress in the game. That causes a very linear path to form. When you're level 5-10, you're stuck here with these items...at level 10-15, you're in this zone with these items...etc, etc. UO, on the other hand, you could go anywhere at any point in the game and attempt to fight whatever you wanted. You could use whatever items you wanted whenever you wanted, and skill progression was organic...not stepped. It was much more fluid and much more enjoyable.
If TESO has a similar sand-box approach, it'll be really nice...although the combat mechanics of TES games are pretty crappy.