These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Arc Fatigue : One of the major criticisms against the relay of the series is that the plot became more episodic and introduced spin-offs with more characters and sideplots (Kriss' redemption quest, a war against a Christian emperor, Louve's adventures, Aaricia's new boyfriend,...) while the main story is still unfinished. For instance Kahaniel's resurrection, announced in the 2007 book, only happens in 2013.
Kriss' resurrection in her spin-off got mixed reviews.
A huge one: Aaricia's behaviour in the third Louve book. She attempts to commit suicide twice when she's driven to believe that Thorgal is dead and then sleeps with her Jerk Ass suitor. For some it was an Out of Character moment for a strong woman like her.
A possibly even bigger one in the 34th album. Thorgal is seduced by the veiled woman he freed from slavery and is strongly implied to have slept with her. Considering how many women Thorgal has rejected over the course of the series out of loyalty to Aaricia (goddesses, noblewomen and Kriss of Valnor, among others), it seems extremely out of character for him to fall so quickly for a woman he barely knows, unless Salouma is more than she appears.
His stubborness and lack of pragmatism could be a flaw. This is made clear at the beginning of Kingdom Under the Sand. Thorgal's insistence to live as a completely free man (meaning in no place ruled by any ruler) meant he and his family spent a long time aimlessly wandering the world in search of the perfect place to live despite knowing the Vikings pardoned him and would allow his family to return. Aaricia points out that the ideal place to live doesn't exist, but it takes a little while for Thorgal to accept this. Heck, he and his family could have led a very comfortable life on the paradise-like island where Arachnea took place...Thorgal's insistence to neither rule nor be ruled was the only reason they left there.
The Fall of Brek Zarith: Shardar is especially nasty, deciding that rather than just surrender his rule to the rightful heir to the throne, it's better to poison his own court and destroy all his treasures, leaving the kingdom bankrupt, hoping that, to restore it, the new ruler will be forced to become as big a tyrant as him, while he escapes and plans to start new conquests, using the psychic powers of the baby he kidnapped.
The Guardian of the Keys: Volsung of Nichor, a Dirty Coward introduced and killed in the third album of the series, is brought back to life by the evil lesser deity Nidhogg, who turns him into a perfect doppleganger of Thorgal. The deal is simple - he will seduce another deity who happened to have a soft spot for Thorgal, the titular Guardian, steal her magic belt, and bring it to Nidhogg, who will return him to his real form and grant him new life. Volsung steals the belt, returns to Midgard, kills Thorgal, takes his place, and, using magic powers of the belt, takes over his village, murdering the current chief, rapes his wife, and threatens to smash the head of Thorgal's baby daughter, all to destroy Thorgal's good name and avenge his own death...that he had brought upon himself - Thorgal's only crime was being there when Volsung got himself killed. He hadn't moved a finger to help him and couldn't have stopped it in any case.
Crack Pairing: Vigrid and the Key Guardian in the second album of Louve. Who would have ever thought?
Fanon Discontinuity: The Kingdom under the Sands is by far the least popular album. Besides this, some fans think the series should have ended with the Qa story, the Shaigan story or when Van Hamme gave up the franchise.
Foe Yay: Kriss shares lots of it with Aaricia (especially the bathroom scene in Kriss of Valnor).
It has also summed up her relationship with Thorgal very well ever since the two met.