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.
Replacement Scrappy: John Davidson constantly tripped over his words, always forgot the "cat's game" rule, and was very often unable to rein in the panelists once they started getting goofy.
Tearjerker: The day after Wally Cox's passing, Peter Marshall opened the show by explaining that due to being taped in advance, episodes featuring Cox would still be seen for the next two weeks. He then added:
Dear Lord, have no fear; Underdog is there.
The same thing happened in the 90's version with the untimely passing of Florence Griffith Joyner.
The Marshall version's move to Las Vegas for it's final season. Besides supposed inferior lodgings (at least according to Paul Lynde), the show's budget was altered as well; no cash per game (each game won a prize, winner of most games won a trip), no Secret Square rounds at all, and the winner of each show would progress in a tournament for a $100,000 prize package ($20,000 cash, an RV, and an ACTUAL HOUSE).
Some dislike the Davidson version for John's ineptness (see above) and the panel's unruliness.
The Bergeron version once used a Bonus Round where the contestant chose a star and attempted to answer multiple-choice questions. In most shows, that is fine; this is a game where, as a contestant, you either agree or disagree. This bonus round only lasted one season, albeit with a $60,000 win under its belt (out of a possible $100,000).
Of course, the season after is when Henry Winkler took over. A lot of things changed that didn't need to be changed (like the contestants having to stand), and the show only lasted two more seasons before it was cancelled.