To begin with, I for one really like a lot of the bold design choices for this game - bold compared to the industry standard (WoW). The decisions to make 100% of the quests content interesting and relevant story-wise, to reduce the amout of information available in the UI so that my screen doesn't have to look like an aircraft *** to make me competitive, to move away from the standard BG/Arenas PvP, to make sure no content even solo is trivial at the appropriate level..
But one of the decisions which I think turns out to be a bad one is the "move out of classes" decision. I was kinda uncertain about it at first, but now I am convinced it's a bad trend (and ESO isn't exactly a pioneer, since it's been advertised by both Rift and GW2 as revolutionary before). The basic concept that lead the first RPG developpers to create classes were to promote diversity and teamwork. The usual problems are that for instance class X is absolutely required (or trivializes) group content in dungeon Y which leads to endless "LFM LAST X FOR Y" that last hours, or that class X design makes is so much better than class Z that noone wants to take class Z in groups / guilds anymore and the players are frustrated.
I believe Zenimax's answer, a dumbed down version of classes where some skills are shared and some are exclusive to a very small numer of classes, isn't solving anything. It's really more about stuffing the issues under the carpet. With fewer classes and shared abilities, you have less obvious class-related problems, but you also have less efficient class-related perks in the game.
This is emphasized by the 5-abilities action bar : 3 weeks after launch, it seems that most serious PvPers already run with pretty much the same abilities in their bars. You end up with 4 archetypes with minor variations. Some skills are absolutely mandatory, some are utterly useless, some are situational but they are shunned away by the mandatory skills. And the problem is going to get worse as people start breaking down the mechanics of the game.
The perks of having a traditional class system is that you shove strengths AND weaknesses down people's throats. If you play a healer, you ARE going to be vulnerable, no you can't get the blink spell to get you out of clipping range within 3 seconds. If you play a destruction mage with glass canon damage, you won't get that PBAoE root. The task of balancing something like that is tremendous, and probably downright impossible. They will never manage to make even 50% of all skills worth being taken in a given situation. Which means that everyone in this given situation is quickly going to learn which abilities to slot. Right now people simply ignore the weaknesses of their class and stack its strengths : Clones wars down the road.
Traditional classes would have allowed for much more interesting design decisions : niche roles are acceptable when they come as a package, they aren't if you can handpick their unique features. Druids in old-times world of warcraft had a very low healing output and terrible resource management, yet it came as the price for their unique buffs, versatility and special perks - like for instance being the only ones able to resurrect someone during a fight. If WoW had been built at that time like ESO is right now, all druids would have simply picked their innervate, buff and resurrect spell and dumped the rest of their abilities into the shared trees, making them grossly overpowered. To solve this, developpers would have had to nerf these "unique" perks, homogenizing and removing flavour from the game.
(Ironically, since that time, world of warcraft developpers have spent most of their time homogenizing classes in their class-based game because they thought people couldn't deal with it, but that's another problem.)