The A.V. Club posted a list of shows in which the lead protagonist is the least interesting of the main characters. It happens all the time, to all kinds of TV shows. The list includes current shows like The Walking Dead and Boardwalk Empire as well as older shows like Barney Miller and The Many Loves of Dobie Gillis. I would add Seinfeld and Bewitched and Northern Exposure and …well, it's such a standard fact of TV that TV Tropes calls it the Designated Protagonist Syndrome, featuring the Standardized Leader. (Warning: following links from TV Tropes can cause you to lose the rest of the day.)
I first noticed the trend when The Mary Tyler Moore Show ended and spun off TV shows for all its other characters. Those wacky characters we all loved then became the main character, and they lost their quirkiness by having to corral a whole new cast of wacky characters. Happy Days, which is on the A.V. Club's list, revolved around the generic Richie Cunningham for the first half of its run. The more interesting character Fonzie was the real draw, but when Richie left the show, Fonzie lost his juvenile delinquent status and settled down with the Cunningham family. P.E. teacher Roger Phillips was brought in as the Standardized Leader, but few remember him as such.
When a lead character is played safe, it is because the viewer is expected to see the action through his/her eyes, so writers can't afford to take too many risks lest the viewer find they can no longer identify with the character. That is especially true if the show was built around the generic lead and the producers hope to make the series last a long time.
My guess is that the inspiration for this list is the success of Breaking Bad and Game of Thrones, two shows that defy the trope in a big way. Breaking Bad very deliberately introduced the main character Walter White as someone the audience could identify with, and then took him into frightening territory to make viewers consider how such a transformation could happen. And the show was always meant to be a story that ends. Game of Thrones is truly an ensemble show. Try to recall who was considered to be the lead character in the series' first season. Both shows prove that the audience can handle having the rug pulled out from under them.
What other shows would you add to the list? Link
In ensemble dramas, the 'boss' frequently got top billing but rarely had the best story arcs: Daniel J. Travanti in Hill Street Blues, Ed Flanders on St. Elsewhere... you could even argue it for Patrick Stewart on Star Trek: The Next Generation until Captain Picard got 'Borged' at the end of season 3.