Well, there are many, many issues with their design - from my point of view.
I like several of their ideas - and I think some of them are actually very successful.
The concept of a dynamic world with on-going events is great, but the execution was less than optimal. First of all, the narrative is all but impossible to follow - which means you're not engaged in the why of it. That's a big problem.
Another problem with dynamic scaling of difficulty - is that you can't finetune encounters. They HAVE to make it possible for any number of players - which means it can never be truly challenging. That makes most of the events feel trivial and samey - because you don't really have to switch up your tactics much at all.
I've actually had the most fun when solo'ing events - because some of them are very hard when you're alone. But solo'ing world events is obviously counter to the intention.
Beyond that - events are highly cyclical - and I don't feel at all like they're changing the world. To me, that's the biggest failure of the execution. It feels like quests that are being reset over and over, and that's just not what I think they wanted to accomplish. They over-estimated their own ability there.
The removal of the trinity was going way too far to solve a relatively minor issue. Yes, having to find a tank and a healer has been annoying in the past - but with the addition of dungeon finder tools - it's almost never an issue. If you play WoW, SWtOR and so on - today - you'll find that the problem is minimal. Sure, if you play off-hours you may have to wait a few minutes, but that's not exactly a tragedy.
The fact of the matter is that there are lots of players who love playing tanks and healers. It's not like they're being forced into it. It may be a little old in the tooth - but it works and it adds an automatic layer of tactics to any encounter, however small and familiar.
The way to innovate here is not to remove the trinity and add nothing in return. They should have expanded the arsenal and skills for the classes, because they're highly situational and don't really match a set role. All classes are DPS-oriented at the core - even if some are better at healing and defending.
In my opinion, the way to expand classes would be to add non-combat tasks - like they do in DDO, where finding and disabling traps is essential. Solving riddles and skill-based challenges is much more interesting than just spamming 1-2 abilities in combat.
They obviously expected to challenge players in new ways - but the end result is a feeling of chaos and it doesn't feel like you're a group working together as much as individuals frantically trying to overcome an encounter. Well, not always - but too often.
The removal of gear-gating was a good idea - but they failed to provide alternative avenues of gameplay. Weapon and armor skins is not new - it's something that goes along with all gear in all MMOs. They should have provided sandbox features - and made crafting meaningful as a way to make money and to stand out. They should have added housing - and I think they should have considered open world PvP, though it would obviously change the dynamic of events.
The personal stories are very, very bad. As in, unbelievably so. What I mean is when you compare them to the other content - it's such a shocking contrast. Almost everything in the game in terms of content is super high quality. Ok, I've yet to play all of the personal stories - but I've seen quite enough to make sure. It's awful writing delivered by awful to average voice actors. This is a mystery to me - and I have no idea how they let this pass.
The arsenal for classes is relatively limited - and character progression is minimal after level 30. That was a huge mistake in a game with a level cap of 80. You DO NOT want the player to feel as if they've seen most of what their character can do so early in the game. This is the RPG genre - and progression is key. Progression is not what they wanted to combat - it was the gear grind and the gear gating mechanisms.
With all that said - I think the combat system remains extremely good - and has such a marvellous sense of flow. The world is beautiful (though I consider it quite lacking in variety - when compared to WoW vanilla) - and everything (except for personal stories) is made with great care and passion. The cities, in particular, are awe-inspiring.
They way they down-scale automatically and makes all content relevant at all levels is very clever - and something that many MMOs could learn from. I love that everything you do in the game feels rewarding - though the nature of their non-power philosophy makes the rewards somewhat less satisfying.
The skins for the legendary items are downright amazing. I'm not that big of a vanity player - but I REALLY want to get my hands on some of those weapons - they look that cool.
Well, I believe I've been through most of that already....