Two reasons why Zenimax dropped the required subscription.
1. The required subscription and traditional MMO financial format was not popular upon release.
2. Consoles already require a subscription in order to play online, they did not want to discourage console players from getting into their game.
Since the change to an optional subscription their game has been more alive than ever.
I was a loyal subscriber even before the plus option but it seems that this current format of using the money store to fund the project has become the most optimal way to keep the game running.
1. Ping below 100ms.
2. Port or ride at speed through any area without load screens, black outlines, and NPCs/monsters just popping into place next to you, or invisibly attacking you.
3. Story mode in the group dungeons, so we can see if they actually have story, not just 3 idiots charging ahead of you then arguing when they die in fire.
4. Riding a mount without butt-riding the ground, or standing on its back.
I mean, I have more, but those would be pretty much "IT'S A WHOLE NEW GAME!"
PVP Everywhere Full Loot and level loss upon death !