For instance, the parent(s) died and someone else external to the family has to step in because "they promised", creating a fish out of water scenario. Think, Gimme a Break (especially after Dolph Sweet passed away), Diff'rent Strokes, Punky Brewster (I know that technically, Punky's mom was still alive, but it none the less, dealt with absentee biological parents), Love Sidney, Full House, The Hogan Family (post-Valerie Harper), and Webster.

"Very special episodes" or focusing on serious issues. Webster, Mr. Belvedere, and Diff'rent Strokes tackled pedophilia, Facts of Life had Natalie lose her virginity, Punky Brewster had an episode about drugs and the Just Say No campaign, Diff'rent Strokes had Kimberly become bulimic, Gimme a Break had an episode on smoking, workplace inequality, and there's the infamous one where Samantha made Joey up in blackface. Family Ties had the Keatons deal with an alcoholic relative, played by Tom Hanks.

"Very special episodes" or focusing on serious issues. Webster, Mr. Belvedere, and Diff'rent Strokes tackled pedophilia, Facts of Life had Natalie lose her virginity, Punky Brewster had an episode about drugs and the Just Say No campaign, Diff'rent Strokes had Kimberly become bulimic, Gimme a Break had an episode on smoking, workplace inequality, and there's the infamous one where Samantha made Joey up in blackface. Family Ties had the Keatons deal with an alcoholic relative, played by Tom Hanks.

LOL! You forgot a few: the "plight of the homeless" episode where almost every TV family had a long lost relative discovered to be living in a shelter, there was also the obligatory "sexual harassment" episode (that one has kinda made a comeback) and the "starving kids in Africa" episode.

Yes "Very Special Episode" is the worst thing about TV from the 80's. When people wonder why Seinfeld with it's rule of "no hugs, no lessons" was such a huge hit, it's because people were sick of this kind of stuff. Sitcoms aren't the place to discuss serious topics, leave that to cop and hospital shows where these topics fit in to the story lines.

Illiteracy

Ahh, yes I forgot that one. LOL!
As much as I love Mama's Family ,it was guilty of three of these: homeless, sexual harrassment, and illiteracy. The sexual harrassment one with Naomi's boss and Thelma was pretty funny though.

Common '80s tropes?

"Family adopts kid near the end of the series' run"

"Mother announces pregnancy in Season Four, has baby that same season, baby is about six by Season Six"

"Third child rarely gets his/her own episodes, is often even written out of the show"

"If family is in living room, the front doorbell will ring. If family is in kitchen, the back doorbell will ring"

"Wacky cousin/neighbor/foreigner joins the cast!"

LOL! You forgot a few: the "plight of the homeless" episode where almost every TV family had a long lost relative discovered to be living in a shelter, there was also the obligatory "sexual harassment" episode (that one has kinda made a comeback) and the "starving kids in Africa" episode.

Yes "Very Special Episode" is the worst thing about TV from the 80's. When people wonder why Seinfeld with it's rule of "no hugs, no lessons" was such a huge hit, it's because people were sick of this kind of stuff. Sitcoms aren't the place to discuss serious topics, leave that to cop and hospital shows where these topics fit in to the story lines.
That's a fair point, but where does fantasy and reality meet? Are sitcoms pure escapism on a Friday night after a hard week or should they reflect the society around us and maybe if possible show a humorous side of what may be a dark issue to discuss?

The best sitcoms do reflect the society around us, but they don't need to shoehorn in a topic of the week. Three of the most popular sitcoms of all time (I Love Lucy, TAGS, and Seinfeld) reflected American culture, but none of them ever got preachy about anything.

"Family adopts kid near the end of the series' run"
This one was big in the 70s, too.

This one was big in the 70s, too.

Very true.

The kids are all perfect well mannered make good grades go on to college the wives are all the trophy model types they drove around in nice pristine cars and lived in good neighborhoods have high end things but always bickered about money even though they have high dollar jobs.

Character has two dates in the same night, and has to pull it off instead of just rescheduling one or the other.

Someone gets amnesia after a blow to the head.

Talent show where we suddenly learn a character is good at playing the piano or singing.

Character gets sick and another character insists on taking care of them while they recover, and gives the invalid a bell to ring any time he or she needs anything. Unfortunately, this backfires because the invalid becomes a pain in the ass with the bell ringing.

Everyone was college educated and worked making six figures and the adult males unless they were 50 or over never served in the military if they did they were screw ups.

Eighties sitcom children crack wise and talk back in a way that would have got them severely punished by real world parents.