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.
This makes more sense if you assume The Movie is the actual depiction of events, since many scenes had to be changed for dramatic tension (eg. Greg writes that he scared kindergartners with a worm, which led to his expulsion from the Safety Patrol, while in the movie he abandoned them in a pit at a construction site.)
Some people take it one step further and categorize Greg as a sociopath. However, it should be noted that he does have occasional Pet the Dog moments not motivated by greed, such as the What You Are in the Dark episode at the end of Cabin Fever where he goes out of his way to deliver a wrapped present to the church toy drive.
Anvilicious: Cabin Fever seems to really really be giving one the point that the author hates the school system.
Fridge Brilliance: Read the books and you will notice that Rodrick, Rowley, Manny, and Fregley are all drawn in a rather unflattering style. This makes sense because Greg doesn't like them and wants to depict them negatively.
However, Rowley is much less attractive in the movie, so it can be assumed that Greg is trying to make him look better
Hilarious in Hindsight: The comic strip Wacky Dawg, stopped being funny when the comic stopped making jokes and was about the dog essentially became a mouthpiece for the opinions that the creator had - sounds kinda familiar when you think about it, doesn't it?
In the 3rd book, Greg said that in the future, everyone will have a personal robot that tells you whatever you need to know. The iPhone 4S, anyone?
In the first movie, Rowley rejects Greg's offer to get ice cream. Fast forward to the next time he ran into ice cream...
On top of that, Rowley breaks his arm. Robert Capron would later play another character in a Haunting hour episode who breaks his arm...and then ANOTHER one who does so.
Jerkass Woobie: Let's not sugar coat it. Greg is lazy, self-centered, a mild example of a Small Name, Big Ego...but he suffers from Middle Child Syndrome, gets picked on by bullies at school and Rodrick at home and Rowley's parents consider him to be a bad influence on their son. It's really easy to see why he's prone to Jerkass moments. Not to mention that little if any of his plans are successful, which in really turns him into a Iron Woobie.
Moral Event Horizon: In the Books and movies, some characters cross the line to the point where Greg's Jerkass personality seems like nothing. A prime example would be when in book six Manny shuts down all power in the house except for his room, so he can be pampered while he leaves his family to die.
In the first one, a gang of teenage bullies make Rowley eat the cheese. Keep in mind, in the book, Rowley was temporarily traumatized by the incident.
Everyone. The print publishers chose to market it as a kids' book but Word of God is that it was written as a nostalgia trip for adult readers "...like The Wonder Years".
Some of the problems that Greg faces involve issues that only older readers would be able to relate to; for example, in the webcomic, Greg loses his progress during the game Twisted Wizard because his mom turned the console off, and the game doesn't let you save. How many children do you think are familiar with older game systems that lack a save feature anymore?