The term Bayformer is a tongue-in-cheek derogatory term used by some members of the fandom in reference to the Transformers featured in the movies directed by Michael Bay. It is usually used in a negative manner specifically toward the choices made in Michael Bay's films concerning the physical designs of the Transformers in the films, as well as their personas, unique attributes, and other various aspects that some claim to make the Michael Bay movies worse than they otherwise should have been. The term is also used in even broader terms to express distaste for the Michael Bay Transformers films altogether.
In short, the term Bayformer originated from disgruntled 'O.G.' fans who think that Michael Bay's image of Transformers, with it's repetitive use of explosions, shallow attempts at drama, sub-par scripting, lackluster characters, emphasis on humans, etc. to be wrong. Really wrong... and that the films would be far better off with more G1 attributes... or, at least, with less Michael Bay. If you desire any further explanation, continue on to understand some of the specific grievances in more detail.
While some fans prefer the more complex designs of the Transformers from Michael Bay's films because they offer a more intricate, detailed and modern appearance, others dislike them because of their perceived lack of individual character. One design element common to the typical Bayformer is that, although still alien in appearance, the Autobots have decidedly more human faces than their Decepticon foes, while the Decepticons are far more reptilian/insectoid in appearance. This is most likely used to make the Autobots appear more friendly, likable and sincere, and the Decepticons more evil and dangerous. Unlike their original designs, the Bayformers have some type of blood and saliva making them appear more alien than machine, and some of them even possesses tentacles.
In most other forms of media that the Transformers have appeared in, such as the first Transformers film made in 1986, the Transformers have presumably had better armor, since human-manufactured weapons such as bullets and rockets didn't effect them in the slightest, whereas in Michael Bay's movies, they damage the Bayformers easily. The weapons the Bayformers use in the films are usually weapons that fire more conventional types of ammunition, such as bullets, rockets, and more recently what may be a shotgun equivalent, while in every other appearance, the Transformers have primarily used either melee weapons, such as axes, morningstars, and swords, or weapons that fire lasers or other forms of condensed energy.
Older fans of Transformers, or those who have watched the original Generation One Transformers Show are more familiar with the likenesses of the original characters now in the Michael Bay movies, and are usually subsequently dissapointed with the way in which they are portrayed in the Bay films, as opposed to their original G1 characteristics. For example, Bumblebee, who was originally depicted in G1 as the soft spoken, kind, and humble underdog, the Bayformer equivalent in the first Bay film can be described as sarcastic, unpredictable and occasionally crude, for the sake of "humor". Others will say that the new films do almost nothing to develop characters in the films (although whether G1 accomplished that in the first place, with it's incessant addition and removal of characters, is questionable). Some fans are also dissapointed with the vehicle forms that the Bayformers take, as opposed to their original forms from G1. Some examples include:
-Optimus Prime, who originally took the form of a cab-over-engine red Freightliner truck, but takes the form of a Peterbilt truck in Michael Bay's movies that was not a cabover. Being as Optimus is inherantly one of the most well loved characters among fans, the changes (most specifically his flame paint job) that Michael Bay's films made to him are among the most heavily and frequently contested. Fans continue to become ever more upset as Transformers 4 unveiled an Optimus Prime vehicle form with ever more gaudy additions.
-Bumblebee, who was a VW Beetle in G1, was changed into a Camaro for the Bay films.
-Mirage, who not only lost his original vehicle form of an F1 racing car to a red Ferrari 458 Italia -a vehicle more suited to Sideswipe, who was instead a silver Corvette- but also was given a foreign accent. This may be the result of inconsistencies with the character, as he was also referred to by the name 'Dino.'
-Soundwave, who -originally being a tapedeck- needed a vehicle form that would in some way display the fundamentals of being able to record and relay information (which was the tapedeck form's primary function), like, say... a News van, or perhaps a drone of some sort, or... a Mercedes SLS AMG. (Yes, he was a satellite for a time- that isn't the criticism, here.)
-Devastator, who was originally a massive, Green and Purple two legged Gestalt Decepticon, was represented in the films as a four legged multi-colored monstrosity, with (again, for the sake of bad humor...) testicles.
Many other characters suffer from similar issues.
Fans also become irritated when Transformers in the Bay films do very out-of-character things, for the sake of a one liner, or action scene, or another theoretically impactful event that ends up being a total buzzkill. A perfect example can be found in Transformers 3, when Optimus Prime is in combat with Sentinel Prime at the end of the film. Remember back in the original film from the eighties when Optimus is fighting Megatron -his arch nemesis? Megatron is nearly deactivated by Prime, and begins begging for mercy. Optimus was willing to spare him, even after he along with his Decepticon cohorts had razed an entire city, killed many Autobots and fought against him in intense combat. However, in the third Bay film, Dark of the Moon, Optimus nearly loses his life to Sentinel Prime, to be saved by Megatron, who feared Sentinel Prime had too much power. Megatron then makes it very clear that he isn't willing to fight Optimus, and that for the time being, they ought to call a temporary truce. Prime then goes completely out of character and kills Megatron, after Megatron hadn't fought him, but saved him. Not only that, but after leaving Megatron in a state of dismemberment, he returns to the now-battered, nearly destroyed Sentinel Prime, and (acting more like the Decepticon he had just deactivated than the Prime most know and love), Bayformer Optimus levels his weapon and shoots the wounded, defenseless Sentinel through the head. Many who criticize the film argue that if there ever was a less "Optimus Prime" thing to do, it would be killing A- Someone who saved your life, or B- Someone who could not fight back.
Others find it upsetting that the Bayformers in the films have to "share the spotlight" with other human characters. For example, in the first Bay film, it isn't Optimus Prime who vanquishes Megatron, but Samuel Witwicky- a human boy. This theme of 'important humans...with robots', rather than 'ROBOTS! (and a few important humans)' annoys fans who argue that a film called Transformers really ought to put more of an emphasis on the Transformers, rather than humans. Furthermore, in every Bayformers film, there tends to be a female character who fills a cookie cutter role- that role being a sexually implicit model who occasionally wears revealing clothing and may say suggestive things. While this may please some people it displeases many Transformers fans, and since the new Transformers films are largely marketed towards children, many view that particular role as unfit for such an audience.
Many fans today have therefore concluded that Transformers under Michael Bay has caused the characters, and even the entire franchise to lose its integrity. People claim that the Transformers characters aren't who they used to be, and that Transformers has become a good film idea, although executed poorly and unauthentically. The new films do not execute the concept of Transformers correctly, nor do they represent the characters in a genuine fashion. To name one example- some claim that in the new films, Optimus Prime is not a true hero, as voice actor Peter Cullen had always intended for him to be. And although the notion that the Transformers show was originally conceived to tell a good story is questionable to say the least, regardless many fans find the near complete absence of any successful references to G1 as unacceptable, especially when the aspects unique to the Bay films are considered, which (many say) do more harm than good.
Fans also become annoyed at the fact that Michael Bay seems to make the same mistakes in every Transformers film he has made. Many thought that after the first film, if Bay was directing the next one, he would hear the pleas from the Transformers fanbase, and make changes. Yet, to this day, the same shortcomings have plagued every, single film. And in many respects, the mistakes and shortfalls seem to have compounded from film to film.
It is primarily for these reasons, among others, that the "Bayformers" films have become increasingly unpopular among some Transformers fans, and hence the term Bayformers was coined.
Oddly enough however, despite the term's derogatory intent, those who are known to like the Transformers from Bay's movies have also been known to call themselves 'Bayformers fans', which is technically an improper use of the word, considering the fact that the word's only true intention is to express hate.