Character Tiers: In a game with as many characters as this, this was bound to happen sooner or later. Characters like Fiona and Karok are valued highly for their high survivability rates, while Kai and Hurk are great at dealing large amounts of damage without much risk of dying. Variants like staff-wielding Evies are a necessity due to their healing skills and solid burst damage. Vella and Lynn don't take hits as well as they dish them out, but are easy to control in the hands of even inexperienced players. Lann is the only character that is subject to any sort of putdown, due to a very limited playstyle that is difficult to use unless by very experienced players (which are rare and few in between).
Complacent Gaming Syndrome: While it's still around, most characters aren't frowned upon. However, Vella and Kai are still much better than others.
This has been reduced to some extent as character balance patches help bring previous characters (such as Lann, Fiona, and Evie) on par with their allies.
Demonic Spiders: The bears of Ben Chenner definitely count, as do the self-destructing Wood Elves.
Evil Is Sexy: The Succubus from Episode 8. Definitely designed to be Ms. Fanservice, since, as in the original game, certain attacks can cause her clothing to break off (and, unlike most other boss break-offs, there is nothing to pick up and no advantage gained, other than escalating levels of fanservice).
That awesome gear you are wearing made from gnoll leather? You are wearing the flesh of sentient beings that you killed just to make it. Especially true with the crimson rage set since the only enemy that drops red gnoll leather is the chieftain who you only have to fight once to progress through the game. Not only that, sometimes the gnolls themselves drop gnoll leather armor; meaning they wear their own defeated enemies as protection too.
The conflict between the Formors and the Humans? Explained as a divine mandated war of genocide initiated by the humans in order to access heaven. The one who tells you this? Tieve, the cute prophetess who believes this blindly even though she admits to not even knowing if the goddess gives a rat's ass about humans. Puts everything in the game into a new perspective when you realize that you are the bad guy.
Game-Breaker: Evie with the Staff update. She can block any attack with mana amber, has insane DPS with powerful spells that have insanely quick cooldowns, and essentially has no trouble soloing any boss.
Not really a game breaker. Loading Evie's really powerful spells takes a considerable amount of time, making them effectively useless against the Lightning Bruiser bosses common to higher level missions from Ainle onward. Mana Amber is very short-lived, and requires almost split-second timing. While Evie is one of the most powerful characters at low levels; starting with Ruins of Sanctity, she has a far harder time soloing bosses than Lann or Fiona.
Kai is a more traditional Game-Breaker in the same sense that bringing a gun to a knife fight is. His shortbow shoots arrows like a semi-automatic pistol (and unlike Evie, he can move while shooting) while his longbow fully averts Annoying Arrows by being outright DEADLY. Couple this with a fairly good dodge and strong special attacks, and you've got a character who can easily deal with any situation, making most of the game trivial.
Mastering the timing of Vella's Cross Cut can allow you to flawlessly make a mockery of any boss geared towards direct melee offense; it allows her to harmlessly dodge through attacks while attacking, and the skill Double Cross allows her to to attack again directly after. As if that wasn't enough, another skill of hers passively buffs the damage of Cross Cut and Double Cross, and all you have to do to activate it is attack a few times.
Windgun Kobolds found in Hoarfrost Depths. While they're weak and slow, their attacks (while weak in and of themselves) carry a powerful debuff that will cause 400 points of damage before it finally wears off, regardless of your armor or defense.
It's Easy, So It Sucks!: The update which added Karok to the NA version of the game also nerfed the difficulty of most bosses. The game's fanbase had a mostly negative reaction to this, as one of the draws of Vindictus is the extreme difficulty of late-game bosses. By contrast, many players liked the nerfing of the Nintendo Hard early-game bosses; as it made solo play possible for many bosses that had previously required insanely well-organized groups to beat.
Player Punch: Ellis, who you spend the first two regions' storylines getting to know, is captured and brutally killed in the fourth Ainle mission, "Wake Up Call," by the mission's main boss, Information Chief Kalis.
Pretty much the entire Episode 10/Season 1 finale. Keaghan goes mad and you have to fight him, Tieve becomes Morrighan, and Keaghan becomes her opposite number.
Rescued from the Scrappy Heap: The introduction of characters beyond the initial four of Lann, Evie, Fiona, and Karok have definitely helped weed out which characters were simply better than others. Fortunately, the developers have started to roll out a consecutive series of character balancing patches, bringing characters like Lann and Evie, who presumably fluttered around the bottom of the DPS charts, all the way to the top, matching and surpassing DPS mainstays like Hurk and Arisha. Similar patches have been made for Fiona and Karok, improving their damage output, while reworks for Kai and Vella are also currently in progress.
While it's notable to point out that character revamps usually happen months, if not years after said character's release, the newest mercenary Sylas experienced such great backlash in South Korea for lackluster damage output and underwhelming gameplay that he was immediately patched to become stronger.
That One Boss: There's a few of these. Most bosses from Ruins of Sanctity onward count if you're aiming for a Solo-Character Run, but even without that, there's a couple of bosses that are outright evil.
Black Breeze is the first case of a truly evil boss. He's a Lightning Bruiser whose attacks are nigh-unblockable, and has a couple of Werewolf helpers that are less powerful, durable, smaller, and more blockable...but otherwise just like it. They fight together.
The Reaper. He's the first boss that combines dangerous moves in both melee and long range, and is fast both in his movements and his attacks. First-time players are almost guaranteed to have trouble with him their first time.
They Changed It, Now It Sucks!: Significant changes to Evie's skills and play style were introduced with the Labyrinth expansion. Player reaction to the re-vamped Evie was about 50% this, the rest being neutral or favourable. The changes to her healing powers especially provoked this reaction, even from those who liked the other changes.