Having a tank doesn't make the encounters harder. But because having tanks and healers makes encounters easier(whole is greater than the sum of its parts), that means the encounter itself can be designed harder.
I disagree. You can design any encounter as hard or as easy as you want. All that the trinity does is give people roles. Raids in WOW are in a large part about giving people something to do. The HC raids have more for people to do and for them to watch out for. There is also a gear wall that needs to be taken into account but that's it.
IMHO, no trinity is more realistic, but that's it. GW2 may not have designed the best encounters (I am not saying that they haven't, I just haven't done all of the dungeons yet so I can't comment) but that doesn't mean that the lack of trinity is a problem. It's just those particular implementations.
For example, Jormag is a far more complex fight than something like Patchwork. All that needs to be done is the following:
1) Give people things to do.
2) Give people various things to watch out for.
That's it. HC raids just add more of the above in some way, shape or form, often with little or no scope for failure. All that the trinity does is it makes designing the encounter easier because you already have a framework (rules) for those two. My problem with the trinity is that it's actually a limiting factor in the fight design. My most enjoyable fights in WOW were where they bent the trinity rules or removed them all together. Think Faction Champions in TOC.
Removing the trinity breaks the status quo and opens up new design opportunities. Have a look at the raid guides on the internet. DPS tasks - Move to position 1, DPS A, move to position 2, DPS B, avoid the green slime, etc. The same for healers and tanks. You can do the same thing without the trinity. You might need to kite a boss around instead of just standing there and blowing cooldowns but it's not that different.
The difference comes with the unpredictability that you get without the trinity. It's like taking away the rules so you can play cards instead of only poker. It allows designers to give the bosses more "intelligence" instead of the high scripting that happens at the moment. That "intelligence" could scale as designers want it to. We may not have the wonderful epic trinity free fights yet but that is not a problem with the lack of trinty.
Which is why GW2 encounters are vastly harder than WoWs? No, they are not. All of the encounters boil down to "kite and res people who are in a downed state."
First of all, nobody has claimed that GW2 encounters are harder than WoW raid encounters; in fact, the opposite has been said (however, if you compare GW2 dungeons to WoW dungeons, including challenge modes and Burning Crusade/Cataclysm heroics, then this is not at all clear-cut; and as someone who plays both, I do enjoy GW2 dungeons as a change of pace on non-raiding days).
That said, your claim that GW2 fights are all about kiting and reviving people appears to be considerably overstating things.
First of all, it just isn't true in general. For example, in CM path 1, kiting is not part of the typical strategy for three of the four bosses. Sure-Shot Seamus primarily requires you to deal with the chain knockbock from blowout (avoid, break stun, stability, etc.); Gunpowder Morgan is a traditional "stay out of stuff" fight; Bloody Victoria is about spreading out for Death Blossom, countering her stun, and removing bleeds. Only Frost really is a boss where kiting is appropriate (and intended).
The issue with the difficulty of these fights (or rather, the lack thereof) is not that you're lacking a tank or that they are about kiting or reviving, but that the mechanics are just very straightforward; there's little mechanical complexity in them. Of course, it also doesn't help that most players are doing explorables fully kitted out in exotics at level 80 when they were designed to be challenging for level-appropriate players (with fewer traits) in blue gear.
Second, even where there's kiting, there's nothing inherently wrong with it; it's a valid form of dealing with aggro as a ranged player. Armies around the world have been exploiting range differentials since time immemorial. If you can avoid having someone get in your face, you do that (see, e.g., longbows at Agincourt). Nor is it really any more or less complicated than standing there and taking hits; it's discouraged in WoW, because WoW is predicated on requiring a constant stream of heals so healers don't get bored, but in a game that is predicated on avoiding damage rather than healing through it, this problem doesn't exist. And even in WoW raids, kiting is often an intended tactic (as an extreme case, half the ICC encounters had adds that were meant to be kited). More recently, Shannox and Garalon were explicitly designed with kiting mechanics in mind (and the use of tanks on Garalon feels thoroughly artificial).
Note that kiting, where it is an intended tactic, can be made arbitrarily more complicated. For example, consider Frost from above: His room has obstacles throughout; he does a stun that you have to avoid or break; and you may have to deal with kiting not only him, but also his adds.
Again, I am not saying that the fight is complex; I'm just pointing out that kiting here is not less difficult than classical tanking (even accounting for the more active mitigation model that WoW has switched to). The fight is overall still simple because the mechanics are still fairly straightforward and its easy to overgear it.
That said, there's nothing that says you can't in principle do more complex fights. The Risen Priest of Grenth compares favorably with some of the easier WoW raid encounters, for example.
Downed state, of course, has been a controversial mechanic ever since its inception. Mechanically, what it does is put you into a state where everybody can heal you (at the cost of drawing aggro and losing a few potentially vital seconds). It is not too dissimilar from standing in stuff in a trinity game and requiring to be patched up (except in this case, it also punishes you by reducing you to your downed state abilities). In many ways, I like the idea better than the healer(s) getting punished for somebody else's failure. It also fits nicely in with ArenaNet's stated philosophy that survival is primarily a personal responsibility.
Now, there are two problems with how downed state relates to encounters. One is that the occasional one-shot attack is unavoidable (or has really hard to identify visuals), putting you into downed state with no apparent fault of your own. While mechanically there's nothing more problematic about that than taking an unavoidable AE or randomly targeted attack in WoW, psychologically it still feels like failure. The other problem is that in some encounters, you never see followup damage in a reasonable amount of time that will actually finish you, nor are their DPS checks that you'll fail as a result, making the fight essentially survivable indefinitely, no matter how hard you fail at the mechanics (as long as not everybody else does, too). However, there are plenty of fights that have neither of these problems.