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.
The Fallen was over 20,000 years old, at least as old as Jetfire (who was falling apart), and would not come to Earth until Optimus was dead, implying that he was somewhat afraid of Optimus, so it is somewhat justified within the movie. Still, it was quite a disappointment to see the guy the comics have always presented as about midway between Megs and Unicron on the Oh Crap-ometer go down so easily.
Alternately, it could be interpreted as a Curb-Stomp Battle. After all, he was doing impressively until a Jetfire-amplified Optimus Prime arrives on the scene.
Alternative Character Interpretation: The scene early in the movie where the Autobots and N.E.S.T. are in Shanghai attacking Decepticons who were doing nothing but minding their own business has led some people to consider that the Autobots (or at least Optimus) are not as heroic as the movie intends them to be seen as.
Badass Decay: Megatron gets this pretty bad; while still very powerful, he's now The Fallen's Yes-Man and is not quite as unstoppable as before. Starscream get this even worse, becoming a complete Dirty Coward.
The scene with the Allspark fragment from the first film falling into the kitchen, turning his kitchen appliances into destructive feral Transformers, a fight ensues, the house is destroyed. Afterwards, this scene is never mentioned again, despite likely being used to establish the power of just one Allspark fragment.
Even worse, after the scene Sam goes right back to insisting he doesn't need Bumblebee's protection anymore.
Critical Dissonance: Despite a 20% Rotten Tomatoes score and extreme hatred toward the film on various movie sites, the film still grossed over $800 million worldwide, and (with only a 2D release, no less) ranks among the few movies to make over $400 million in the U.S./Canada market; indeed, it was the 2nd highest grossing film of 2009 (behind Avatar) in the United States, and the 4th highest worldwide, and adjusted for inflation is the 81st highest grossing movie of all time. It has also received a solid Cinemascore rating and Rotten Tomatoes user rating, and has sold generally well on DVD and BluRay.
Presumably why this movie was hyped as "the worst movie of 2009" to some and beat out Dragonball Evolution and similar worse-reviewed flops at the Razzies.
Never Live It Down: Before the release of RotF, the first Transformers movie was pretty well-liked but still debatable (even being nominated for a People's Choice Award). After the release of RotF, people started lumping the first movie together with its sequels as one of the "worst action movies of all time", and generally feel that any sequel released will inevitably fall into the same pit of putrid quality (including the two that have already been released).
While this is largely considered the worst Transformers live-action film by critics and a good portion of the fanbase (due to its slapdash nature, over-used humor and underdeveloped plot), a good number of Transformers fans consider it to be the best of the trilogy, owing to it being the only one that feels like Transformers (The first movie had a far greater focus on humans - to the point of the Transformers almost being a secondary plot, while the third felt detatched from the mythos, changed several characters' core personalties and felt more like a Star Trek movie to some people).
In retrospect, with its lower-quality writing and cheesy plotline, it does feel a lot more like the old 80's cartoon.
Memetic Mutation: Optimus' line "Give me your face!" right before tearing the Fallen's face off.
Misblamed: Some accused Michael Bay of holding an anti-Barack Obama agenda, since he is specifically mentioned as being the president, and the government officials go from being generally cooperative (albeit in over their heads) in the last film to being complete jerkwads in this one. However, as Bay pointed out, the screenplay was completed and filming began at the start of 2008, before he or anyone else involved knew that the Democrats would nominate Obama (Hillary Clinton was still the favorite for their nomination at the start of 2008, albeit by a slim margin), much less that he would out-poll all other presidential candidates. Additionally, Obama is only mentioned in a brief line that was clearly dubbed on in post-production, which could easily be swapped out for one mentioning John McCain with no real effect on the movie.
The embarrassing ease with which Optimus is able to hand Megatron his ass right before the forest battle, while trash-talking him, is this to a lot of people. Then, during the otherwise spectacularly rendered forest battle itself, there's a very glaring animation shortcut, where, for a split second, Megatron goes from charging furiously at Optimus to strolling complacently. It's partially obscured by the nature of the shot, but many sharp-eyed viewers found it hilarious.
The hilariously awful "Give me your face!" line.
Never Live It Down: So far, the franchise hasn't really been able to escape the negative backlash generated by this movie.
There's debate over whether Skids and Mudflap were really that bad — many people point out that if Michael Bay actually possessed a decent sense of humor, they could have been Actually Pretty Funnyandbadass. As it was, all their humor was based on derogatory stereotypes.
At least some of those are countered in the novelization. For example, Skids' and Mudflap's line that "They don't do much reading" to justify why they couldn't read Cybertronian symbols (when they're, you know, robots who have computers for brains) is changed to that they can read, but the symbols are from the language of the Primes, which they don't know and is separate from the standard Cybertronian language.
Though he certainly looked awesome and had Tony Todd's performance gong for him, between having zero motivation or characterization, a bizarre grab bag of unexplained powers, an awful, anticlimactic clusterfuck of a fight to his name, and nothing shown in the way of actual transformation, there's a reason the Fallen himself is widely considered to be the worst Big Bad of any of the four films. Even Bay himself admitted that the Fallen was "kind of a (expletive) character" in an interview.
Any and all of the humans (barring maybe Simmons, Lennox, and Epps), but the worst offenders would have to be Sam's parents and Leo.
Seinfeld Is Unfunny: When ROTF was released, critics hated it, but general audiences were kinder to it and even defended the movie. Now, after twosequels full of deliberate attempts to correct ROTF's flaws, the movie is far less popular with general audiences, and is much more widely hated for turning the Transformers series into the critics' favorite punching bag.
Other quality issues aside, this movie does contain the famous forest battle, generally considered to be the series' best fight scene.
Whatever you may think of Devastator himself, his introduction falls squarely under this.
Sophomore Slump: One of the biggest examples in film history. Just try and find anyone who prefers this movie over any others in the series.
Special Effects Failure: It's plainly obvious that the rendering and animation of the robots during all of the desert scenes, with the notable exception of Devastator, aren't up to par with the rest of the movie. The scenes that take place on Saturn's moon and in particular, the one where Starscream backs into a hatchling's pod, also look unfinished. It's particularly unforgivable when you consider Revenges budget outstripped its predecessor's by $50 million. Appropriately, while the first and third movies were nominated for the Visual Effects Oscar, this one was not. See Narm above for another infamous example.
Would you imagine that the script was only assembled in 3 weeks? Somewhat unavoidable, due to the writers' strike going on at the time.
A better example is how Michael Bay (or whoever was in charge of release schedules) insisted that the movie go ahead despite the writers' strike.
The film literally reuses a shot from not just the first film (the Autobots arriving at Earth being used for Decepticon arrival) and one from literally an hour ago (Soundwave in space relaying Decepticons orders). They also utilized a shot from Pearl Harbor (the scene in which ships are shown), though this is less noticeable.