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.
Actor Shipping: Quentin×Mélanie, popular even before the actors were known. A bit different from other examples in that they are actually currently dating (and caused quite an uproar when they revealed it!).
Base Breaker: Laura. Some fans are glad a new Lyoko-Warrior finally is joining, others (apart from the Die for Our Ship aspect) are questioning the point of adding a feminine counterpart to Jérémie when such a character (Aelita) already exists, while still others would rather have had Sissi as a new recruit. Not helped by "Virus" giving her major Common Mary Sue Traits.
The 2D being replaced by live-action, of course. Half of the fanbase is excited while the others are crying the death of the series. Whether the series should have even had a new season is a Base Breaker.
The fact two of the Lyoko sectors, Ice and Forest, were suddenly erased for no reason wasn't well-received by some fans. Others seem to be ok with it, thinking it will be properly explained later on. As of the end of the season, it's not explained properly in the show, but interviews with the developers brought to light that they decided to leave out the Ice and Forest sectors so that they can increase the quality of the other projects they adapted for Evolution. In other words, making two really nice sectors instead of four mediocre ones.
Fans are divided about weither Laura being excluded from the group and getting her memory erased at the end of "Mutiny" was a well-deserved punishment or a Hypocrite move from Jeremy, considering how many times he himself ended up putting the Lyoko-Warriors in trouble with his own mistakes.
Canon Sue: Laura has shown many signs to be considered this. She shows up out of nowhere as new student who proves to be able to outsmart Jérémie in her first episode. When she returns, she proves capable of designing a virus that will work on a Lyoko-like system which even Jérémie himself could not do, and in the process figured out the existence of the Return to the Past and how to exempt herself from it, doing the latter right under Jérémie's nose while simultaneously working on the aforementioned virus. Whenever a problem that Aelita and Jérémie can't solve shows up, they just call for her, and she solves the problem in a few seconds, while calling it "easy". The only thing that might save her from being one is the fact the Lyoko Warriors are not at all happy with the way she's integrated herself with them and only include her out of necessity. Finally subverted in spectacular fashion in "Mutiny", where she makes her last mistake.
Die for Our Ship: Laura wasn't well-received by Aelita×Jérémie fans when it was revealed she would serve as The Rival to Aelita. And it only got worst after her introduction episode.
Fanon Discontinuity: It's already been getting a lot of this. That a new creative team is working on it, with the old one only there for the first two transitional episodes, makes it even easier.
Informed Wrongness: In "Vendredi 13", William and Ulrich get called out by Yumi and Aelita for acting "dangerous, idiotic and unwise," simply because they decided to pick a fight with a couple of Mantas while the girls deactivated the tower. Aside from the fact that this is a perfectly sound strategy to handle the situation, they weren't even outnumbered and were working well together. The only thing that could be said against them is that they perhaps weren't taking the situation as seriously as they should have been, letting the Mantas strafe them a couple times while doing a Nonchalant Dodge. Yumi and Aelita come off as being overbearing rather than reasonable as a result.
Introducing a spectre in the form of a child who, rather inexplicably, attacks the heroes with easily escapable (literally, they just shove the kid away) yet somehow debilitating hugs. Given, the spectre drains energy from the Warriors by way of skin contact, but it still looks silly. A William spectre does it in the fifth episode, but he was at least strong enough that it took Ulrich's intervention for Yumi to get free, and he later (offscreen) broke down a door.
Even more ridiculous in "Soupçons", where XANA's attack consists in summoning three clones who... stay in front of a door doing nothing to attack the Lyoko Warriors (who are right in front of them) until Aelita deactivates the tower. Even more ridiculous, one of them uses his lightning to repair a stereo, for no apparent reason. In the end this attack ended up helping the Lyoko Warriors more than it hurt them.
Really, almost any attack could count as Narm, considering most of the time they don't do much more than being a nuisance, yet the Lyoko-Warriors react like this was a big deal.
Special Effect Failure: Unfortunately, XANA's attacks on Earth could be more convincing. It's not helped by the fact that the replacement of 2D by live-action forces XANA to give up his usually spectacular supernatural phenomenon (weather control, Attack of the Killer Whatever...) for attacks easier to adapt (spectres).
Take That, Scrappy!: Laura has her memory wiped after betraying the group and essentially gets booted off the main cast.
The 2D being replaced by live-action is the most obvious examples, but it's far from being an isolated case.
Believe it or not, there were fans who were mad the Supercomputer wasn't exactly identical to his animated counterpart (such as reactivating it requiring to connect to cables together instead of drawing a switch).
The new theme song is considered by many fans to be less unique and less likable than the original.
The fact all Lyoko Warriors can now deactivate towers instead of Aelita alone.
The redesign of the Lyoko Towers to look more like something out of TRON still doesn't sit well with some fans.
Several small elements that had to be changed or removed due explicitly to the limitations of the live action: the appearance of the manhole cover note likely constructed so that the actors would have a place to climb into given that those scenes were shot on location, the removal of Kiwi and several other minor characters, the aforementioned Special Effects Failure...
The removal or drastic change in appearance of minor characters — for instance, Sissi and Anthéa are now blonde.
The spectres in the original series had Shock and Awe powers, were Made Of Air, had Super Strength, and were basically unstoppable. In Evolution, a child spectre has trouble getting through a door, isn't strong enough to restrain Aelita or Odd, and is generally not that big a threat. A spectre of William was only mildly better, being able to restrain Yumi (but then got thrown off by Ulrich) and could actually break down a door. And let's not even talk about the infamous attack in "Soupçons". Their abilities have improved with each episode, suggesting that XANA can't do better at his current power level and they gain more abilities as he gains more strength.
Sissi could be another example. While she was a considered a major annoyance in Code Lyoko, she's completely out of the picture so far. Justified in that she's on friendly terms with the gang now (verbal sparring with Odd aside) and has no reason to antagonize them.
XANA himself. The original series had him conducting massive attacks, such as invasions of killer rats, wasps, birds and wolves, a Zombie Apocalypse, natural disasters, and hacking a military Kill Sat, among others. In this series, in addition to his crappy spectres, which have the excuse of not being allowed to kill the Lyoko Warriors, he's conducted attacks that have served literally no other purpose than to waste the heroes time, such as bugging up the cell phones or the school's computer network.
In terms of the monsters, the Tarantulas now shoot Eye Beams from their head instead of their arms. While this aids in mobility, it's made them fairly weaker than they were prior.
A debatable case with the Scyphozoa: in the Original Series, it was nearly indestructible, to the point it was only destroyed once in the whole show (and even then, it was more hinted than anything). In "Mutiny", it takes significantly more time to take over William's mind than before and is easily destroyed when the Lyoko-Warriors manage to destroy its tentacles. Some fans, though, consider this a case of Tropes Are Not Bad, since the Scyphozoa was pretty much a Boring Invincible Villain in the original series.
Lampshaded by Ulrich toward Odd in "Spectromania": if he had REALLY been the spectre, he wouldn't have waited so long to take Odd's code. Odd would have made a much smarter choice sticking with him so he wouldn't confuse them instead of leaving him behind. In Odd's defense, though, he was in a state of panic and barely recovering from a previous spectre attack, so his irrational reaction had some justification.
Jérémie in "Virus"; despite being there all day to watch Laura when she is conceiving the virus for them, he fails to notice she has been checking the Return to the Past codes at the same time and modified the virus so it would protect her from it. Keep in mind the previous show portrayed him as a genius who could come up with complex programs, and that he has several years of experience with the supercomputer, while Laura is supposed to have none.
Aelita in "Rendez-Vous". She blindly accepts a message promising her information about her parents if she comes alone to an isolated location when she knows XANA is attacking. When she goes, she finds her mother acting so suspicious she'd have to be shooting lightning to be more blatant. Then Jérémie arrives to get her away from the spectre... and she accuses him of lying, unable to accept the fact it couldn't be her mother. After the spectre is destroyed, she blames Jérémie for it, though she at least realizes how stupid this is by the end of the episode.
Laura gets one when she decides to show the lab to her father just to convince him not to remove her from Kadic. At least Sissi had the excuse of being panicked and worried for Ulrich when she did the same in the original series' prequel. Laura should know they could just use the RTTP to fix things (unless she was paranoid they wouldn't just to be rid of her). Naturally, this result in the Lyoko warriors losing even more of the already limited trust they had in her.
In the same episode, Laura's father is more interested in the supercomputer than he is in the fact that a group of teenagers have access to a nuclear-powered supercomputer. Skewed Priorities, much?