Having Open world HNMs and Massive PvE battles does not make up for the elimination of all Freely explorable zones. Having a series of instanced content that has enterence requirements is all well and good. Sometime, if done right(i.e. Nyzul Island) it can be very fun and challenging. But the simple fact remains that Instanced content becomes stale significantly faster than open world content. Open world dungeons allow for Community created fun on top of a significantly larger opportunity for developer made content. You can have a larger variety of content in an open world dungeon, as opposed to the Speed Run/Boss Battle content you get from instances.
When you remove things like timers you gain free exploration and the ability to really take in the zone. When is the last time you actually took a look around Toto-Rak or Darkhold? Probably never.
When you remove maximum group requirements you are no longer forced to tell friends they cannot join because there is no room. People say this makes Boss Battles less challenging because you can just throw bodies at it. I say, it opens bosses up to becomes Significantly more challenging with godly levels of strength that require massive parties. On top of that, for more standard bosses, you gain the ability to low man battles which requires significant skill and grants bragging rights to those who can complete major challenges with minimal assistance.
Finally, with open world dungeons you gain the ability to meet people in the field. MMOs are supposed to have a community. Players are supposed to happen across one another. when you remove that aspect from the game you minimize the ability of the community to grow and develop friends and rivals. Worse yet, you end up with a game in which players actually have to go online and apply for Friends on some LS's website. Yes, this allows people to weed out obnoxious player, but what you lose is the chance to happen upon a friendship with the thousands of players in the games world.