The tagline of Shrek 4 was "THE WAIT IS OGRE!" As a result, mentioning Shrek on /co/ will often result in a Hurricane of Ogre Puns. The most notable example is this fanart getting captioned with "I'm sorry Shadow... but my time here is ogre."
Seen in the flashback in the alternate-universe when Rumpelstiltskin is shown completing the deal with Fiona's parents to make "their problems go away", doing so by making them go away. That would have also happened if the messenger had arrived a few seconds later at the beginning of the movie, but we don't know that at the time. When we see the king and queen's looks of indescribable horror as they realize they're being erased from existence, Rumpelstiltskin crosses the line from just a simple trickster who wanted to rule the kingdom into utterly, irredeemably evil.
Sequelitis: Fans are divided between whether Forever After is just as bad as Third or better. Reception has been much warmer for Forever After than for Third, however.
So Okay, It's Average: The general consensus from fans is that while it's a watchable movie that improves on Shrek The Third, it doesn't have enough of the magic or humor of the first two movies to be worthwhile.
They Wasted a Perfectly Good Plot: A charge thrown by some at the fourth film, which presents a Bad Future wherein Shrek was never born and never met his true love Fiona. It would have been interesting if someone else was Fiona's true love and seeing Shrek deal with that, but Fiona loves Shrek regardless.
We also never find out what happened to Lord Farquaad in the alternate timeline, since he likely never died.
Tough Act to Follow: Zigzagged with Shrek Forever After. As it came on the heels of the poorly-received Shrek the Third, it wasn't hard for it to improve on its predecessor. However, it also only came a few months after How to Train Your Dragon, DreamWorks' best-reviewed movie to date, and also had the first two Shrek movies to live up to.
Unintentionally Sympathetic: While Shrek storming out of his own children's birthday party (after smashing their cake) isn't exactly a good thing, he was going through tons of stress at the time, with everyone demanding things of him, the pigs having ate the first cake, and nobody letting him have a moment of rest. With these factors in mind, Shrek losing his temper is a lot more justified.
What Do You Mean, It's for Kids?: The film gets pretty dark sometimes, people. From the malicious Rumpelstiltskin who erased Fiona's parents from existence and murders people (even his own followers) for his own amusement, to the Crapsack World that AU!Far Far Away becomes, there are some dark themes explored in this film.