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.
Badass Decay: Shrek himself, which was noted by several reviewers. In the first movie, he curbstomped Farquaad's knights. In the this one, he's much softer, and didn't do much to fight back against Charming. Shrek gets some of his badassery back in Forever After, but is largely overshadowed by the Proud Warrior Race Guy ogre tribe.
Puss and Donkey's cover of "Thank You (Falletinme Be Mice Elf Again)"
Filler/Padding: The entirety of Shrek the Third could arguably be considered this in relation to the series as a whole. The film has no major plot events that affect Shrek Forever After (aside from witnessing the birth of Shrek and Fiona's children) and none of the newly introduced major characters even appear in the subsequent film. Seriously, try watching Shrek, Shrek 2, and Forever After in that order while skipping over The Third. It has almost no bearing on the other 3 films.
Genius Bonus/Parental Bonus: The two birds that perch on Snow White's shoulders just before she sics all the forest animals on the evil trees? They're woodpeckers.
Heartwarming Moments: As weak to the franchise as Shrek the 3rd was, I find it quite satisfying how it manages to continue to subvert typical fairy tale endings. The case being how, with the exception of Prince Charming, the villains he enlisted realize fromArthur's speech that they can get their own happy endings, from their rather odd jobs that made them happy in the first place. Before, Prince Charming played on their own universal sadness to get back at the happier folk (like Shrek stealing his arranged princess), but they all decided not to be so mean about their Freudian Excuses anymore and be who they really are inside. As a movie series, Shrek certainly knows how to bring it close to home.