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.
Adaptation Displacement: Those who fondly recall the original see the Narz/Trebek versions as altered aberrations, while those who grew up on the Narz/Trebek shows see the original as dull and stodgy.
Game Breaker: Whenever a contestant solved the puzzle after making a match on the game's first turn. When this happened on the original NBC series, and then again early in Classic's run, it was woe unto the unfortunate contestant, who was immediately herded offstage with parting gifts and nary an opportunity to play. A few months into Classic, this was averted with a best-of-three format and the first game's loser going first in the second game.
Replacement Scrappy: Ed McMahon, who replaced Bob Clayton in March 1969 at NBC's insistence, was ousted after six months because he apparently couldn't grasp the format. Norm Blumenthal says the real reason for McMahon's removal was ratings, which took a nosedive when Clayton was replaced and rose again when he was reinstated.
That said, as a photo here demonstrates, McMahon seemed to have good memories of the show — he posed with fellow Concentration hosts Jack Narz and Alex Trebek (plus a Steve Ryan rebus) to help celebrate 50 years of game shows.
Surprisingly Improved Sequel: The 1985 pilot and Classic Concentration used a computer-generated board with 25 panels, which was not prone to the same mechanical failure as the old board...although a glitch was eventually discovered in 1991; see Throw It In on this show's main page.
The Narz era added "Extra Number", which when matched allowed the contestant to pick three numbers on the next turn (for the final season, picking three numbers became regular gameplay), and "Free Look" automatically exposed that part of the puzzle. In addition, the main-game prizes were formerly the parting gifts given during the tail end of the original series.
The 1985 pilots with Orson Bean (there were five taped that day) had contestants matching related words in the main game, with $100 per match. Considering how words can connect in myriad (and sometimes obscure) ways, using predetermined "connections" was a Standards & Practices nightmare waiting to happen (imagine a contestant, after being told the two chosen words were not related, who immediately proceeds to give a sound, logical connection between said words right on-camera; what do you do?).
Classic returned to prize matching, but had an absurd amount of rule changes. One of these, forcing contestants to retire after winning a car, caused a What an Idiot moment near the end. note (It's baffling as to why they called it "Classic", though, when it only resembles the 1958-73 original in its basic objective. It may be that Classic was added for Trebek's pilot to clarify that they returned to prize-matching, and the name just stuck.)
Lots of misguesses, such as "The Pounds You Lost Tonight" (1974) and "Polly-Wolly Tear Doll" (1978). The correct answers were "The Way You Look Tonight" and "Polly-Wolly Doodle", respectively; for the latter, Narz explained that a droplet on an eye is "Tear" while a droplet on a leaf is "Dew".
Orson Bean. For some reason, he felt an absolute need to read out each prize's name during the 1985 Bonus Round.
Ben's moment of stupidity on the fifth-to-last episode of Classic.