9. So Much Flanderization
Shows evolve as time goes by, that's just the nature of things. Whether that be the way they're shot, written, or even the characters themselves, a good series knows when to take things in a different direction.
However, one thing that can feel a little jarring is when a specific characteristic or trait of a personality suddenly gets shoved to the forefront of their characterisation to the point where they simply became a caricature. In other words, flanderization - the term named after The Simpsons' Ned Flanders, a character who went from a moral Christian and all-round good neighbour to over-the-top "bible-thumper".
In the case of Family Guy, this can be seen in the growth of characters like Quagmire, with his creepy and horny antics seemingly becoming more and more heightened and disturbing as the show runs on. Meg is another example of this, going from run-of-the-mill, misunderstood teen daughter to perennial butt of the jokes within the family.
These drastic alterations over the course of the show's life may not stick out upon watching its modern offerings, but after a glimpse at Family Guy's formative years, they stick out like a sore thumb.