Bayformers is a tongue-in-cheek description used by certain members of the fandom in reference to the Transformers of the 2007 movie. It is usually used in a derogatory manner toward Michael Bay and the physical designs of the Transformers.
Most of the Bayformers have a lot of parts that spin. Megatron the Pineapple-former is a typical example of the Bayformers. One design element common to the Bayformers is that, although still alien in appearance, the Autobots have decidedly more human faces than their Decepticon foes, while the Decepticons are far more reptillian/insectoid in appearance. This is most likely used to make the Autobots appear more friendly and reliable and the Decepticons more evil and dangerous. Unlike their original designs, the Bayformers have some type of blood, and saliva making them more alien than machine. Some of them even possesses tentacles.
Besides, since the Transformers evolved/were created on Cybertron it makes sense for them to not resemble humans.
While some fans prefer the more complex designs of the Transformers from Michael Bay'films because they offer a more alienized robots, others dislike them because of their lack of individual character and attachment to previous versions fans are familiar with from older franchises, mainly making them look like aliens instead of Transformers.