Perhaps the two most famous recent examples of poor cast additions are the recent spectacular failures named Nikki & Paulo and Maya & Alejandro. Both sets of characters entered pre-existing universes with their own isolated stories. That being said, there were a few differences between the two pairs.
Nikki (Kiele Sanchez) and Paulo (Rodrigo Santoro) were introduced in the beginning of the third season of "Lost." The attempt to introduce new regulars (who in theory had been there all along) was admirable, but it ended up failing nonetheless. While their final episode was intriguing (showing past events from the perspective of the "outsiders") it ended up being too little, far too late. Nikki & Paulo were introduced very sloppily, and their appearance seemed completely out of place. They never really meshed well with the rest of the cast, and it didn’t seem like they belonged. The fact that they were both exceedingly unlikable characters did not help very much either. Thankfully, their deaths soon followed, and they have been all but forgotten in the wake of more successful character introductions in season four.
Maya (Dania Ramirez) and Alejandro (Shalim Ortiz), were two of the several new characters added in season two of "Heroes." Their storyline, however, was wholly uninteresting and almost completely isolated from the rest of the series (at least for the first few episodes). Because of this, their inevitable appearances quickly became dreaded by fans of the show, and their scenes were probably skipped over more often than the commercials by viewers using a DVR. Unfortunately, while Alejandro is dead, Maya lives on to threaten the world with more wooden acting in season three.
"Happy Days" had a large and somewhat fluid cast throughout the years, as comings and goings became quite common. However, as the series lagged on, the goings included popular characters played by folks such as Ron Howard and Donny Most, while the comings were less than compelling. wo such additions in later seasons include Chachi (Scott Baio) and Roger (Ted McGinley).
Chachi was a horrible character and had little chemistry with his paramour, Joanie (Erin Moran). In fact, "Happy Days" is 100% accountable for two tragedies related to the Chachster: making Scott Baio a household name and the eventual creation of the abomination otherwise known as "Joanie Loves Chachi."
Then there is Ted McGinley. McGinley has built himself a reputation over the years (a not altogether fair one) of being the guy whose arrival signifies the beginning of the end for a series. In some cases, that distinction is questionable (he hardly ruined "Married... with Children") but in the case of "Happy Days," it is spot on. His character, the jock Roger, was a completely unnecessary addition to the ensemble cast and contributed nothing in the several years that he appeared as a regular. Surely, the demise of "Happy Days" has more to do with the departure of Ron Howard than the addition of Ted McGinley, but it sure is a lot more fun to blame it all on ol’ Teddy Boy.