From what we are seeing on PTS Arcanists skill line dedicated for DPS is simply the best dps spec. It has not only some of the "best in slot" abilities but also the best passives for any dps spec. Lets take a look
Passives
Fated Fortune - Crit dmg
Harnessed Quintessence - Weapon/Spell Dmg
Psychic Lesion - Damage and chance of status effects
Splintered Secrets - Penetration
Skills
Fatecarver - AOE class "spammable" skill which hits almost as hard (if not harder) as skill dedicated to do single target damage. It hits harder than ultimate. Morphing allows for utility in form of damage shield or longer channel/more damage. With most of the fights being AOE fights it is the best spammable in the game.
Tome-Bearer's Inspiration - Probably the best source of Major Brutality and Sorcery in a game, class abilities deal extra damage
Abyssal Impact - Usually people pay attention mostly to "beam" but this is another great ability from this skill line. It hits really hard and it is aoe. You deal % more damage to enemies effected by Abyssal Ink
The Unblinking Eye - One of the best aoe ultimates. This one can be controversial. It competes with skills like Meteor and Elemental Storm. With morph to increased damage every 0.5sec it actually makes up for shorter duration and does similar dps. Ring is at the disadvantage of being shorter radius however destro ultimate requires 75 more ult points. Meteor on the other hand is similar cost (15 ulti point more than ring) with less damage unless it will be able to hit multiple targets to regenerate it very quickly.
The Imperfect Ring - Class DoT. Another ability with great dps potential. Even in single target it does decent damage but shines in aoe.
Runeblades - this is the only "bad" ability in this skill line. Well, it is not actually bad but the main problem is that it was designed to be single target crux consumption ability for arcanist however if you use it over beam you would actually lose dps even in single target.
Arcanist has all of his tools in a single skill line. Some of them are simply BiS for DPS role and what makes it even worse is the fact that it is incredibly strong just on its own. You really won't sacrifice much if you decide to replace 2 other skill lines.
Now, lets take a look at some "iconic" abilities and associated skill lines from other classes which will compete with Herald of the Tome for DPS:
Sorcerer (Dark Magic)
Crystal Frags - Single target damage ability, can be used as spammable as it does good damage. Even better if it procs for instant cast and more damage. It also provides Minor Prophecy for your group with exploitation passive
Other skills are either situational or completely skipped. Passives are mix of improved sustain and survivability
Dragonknight (Draconic Power)
Burning Talons- one of core DoT abilities used on Dk dps.
It is the only mention from this skill line. Everything else is dedicated for different role.
Warden (Winter's Embrace)
Winter's Revenge - Great AOE dot. Used on every Warden DPS
Arctic Blast - AOE ability sometimes used on Magdens
Passives: Extra damage with frost abilities and extra damage/chance with chilled status effect
Templar (Dawn's Wrath+ Aedric Spear)
In my opinion Templar is the most well designed class in a game. If every class had templar design subclassing wouldn't be an issue. It is complete opposite to Arcanist as it has its power spread across all 3 skill lines (well, for dps mostly 2 of them) and combination of them make it powerful as a whole. Unfortunately Herald of the Tome provides as much power as Aedric Spear and Dawn's Wrath combined. Keep in mind these are separate skill lines and each of them will compete with different classes.
I'm fully aware that I mentioned skill lines which have 1-2 abilities dedicated for DPS and generally they are better suited for different role . Also, we have skill lines like Nbs Assasination. Wardens Animal Companion or Necros Grave Lord and they are also dedicated for 1 role only so what am I talking about?
Well, my point is that Herald of the Tome is incredibly strong skill line in any scenario. From what we have seen on PTS Herald of the Tome is completely dominating subclassing as it is simply too powerful to be skipped. The main theme became to take Arcanist and use subclassing to boost its damage even further. Arcanist beam builds are being challenged only in single target fights by combination of Nb, Sorc and... Arcanist.
Herald of the Tome is used in every scenario, either as core of the build with beam or damage buff to a different spec. Sadly there are some classes which are getting completely skipped since they either don't provide enough buffs boosting your own damage or can't compete with Arcanist damage output.
I'm not here to ask for massive nerfs to Arcanist. It would only hurt those players who decide to pass on subclassing (which should also be an option) and they should not be punished for it. Instead of nerfing it I think devs should shift some of the Herald of the Tome power into different skill lines. This way Arcanist will be string powerful just on its own but not too powerful to make it outshine everything else.