Giving a class powers that was not intended/available thrue items, is a problem, and it was something SoD was doing alot (and still does, Jyre for example).
It does make it fun to play a class with the ability to do something that was not really its role.
Imo gate clickers are in the same category, but all it does it shorten "down time", it doesnt help win a hard fight.
One spell can also redefine a class.
Like a Necromancer with out his Archaic, is a bad Necromancer. His primary role is DPS, and he probably gets upwards of 50% of his DPS from his Archaic (in a group where mobs dies fast, he would for sure do twice the dps of a necro with out Archaic).
Druids with R2 is a whole other Druid than one with out.
If Shaman R1 was worth casting, deff a different class too.
Beastlords with R1 also brings a whole different level of DPS.
Tomes, you could say the same about tomes too.
Like the Mage one that makes their pet goes insane with 12k hits or what ever that was all about.
Can't really think of any other insane tomes, perhaps bards Blademaster's Rhythm, 10% chance of 8 extra swings, seems like a 80% melee dps increase to me (in 10 swings, would proc once, and do 18 attack and not 10, 80% more melee dps. Usually Bards do atleast 50% dps with their songs though.).
Nerfing healers ability to DPS thrue items, is probably the most obvious of these "1 item/spell/tome that change the class completely" problems, but its not the only one, not at all.