Herschel's Farm.  Getting stuck on the damned farm ruined The Walking Dead.  I mean, it wasn't on strong legs to begin with, but the farm killed it for me.

The original "Shark Jump" signified a point of no return for Happy Days though, whereas this was simply a low point.  In my opinion, The Walking Dead went on to redeem itself many times over.

I'll have to take your word for it that the series redeemed itself, as I stopped watching at the end of the farm season.  I'll even grant you that the original Shark Jump signified a point of no return.  But I would argue the scope of meaning includes a general loss of the original vision.

Happy Days continued for 7 years after the Shark Jump episode.  I have to assume people still liked watching it, but consensus is that it changed at that point to something less than it was.

Granted I haven't watched all of Walking Dead, so I can't speak to the quality, but I still think it changed from the original vision.  It seems to be the crew goes some place (eg. a farm, or a prison) for a while (half a season or so) and enters into conflict with the inhabitants already there.  That's certainly different from a mismatched group of people on the move struggling to survive in a Zombie apocalypse.

Do you think the series jumped the shark at another point?  I hear the current season seems to be dragging...

I think the current season is in danger of a shark jump, as they've implied that a main character was killed in one episode, but three episodes have since passed, and it still hasn't been confirmed.  there's a strong possibility that he escaped his predicament, but I have a difficult time imagining how that might plausibly have happened, even with a generous dose of suspended disbelief.  Still, I'm giving the writers the benefit of the doubt that they'll have come up with something good.

Aside from the suspense over the fate of that character, I think the season has been good: there has been some decent character development, and an excellent back-story episode for one of the characters that hasn't been seen since the early seasons, but has returned with a much improved outlook on life.  The typical (approximate) formula for the show is to come out swinging early in the season, then spend several episodes on story development, and making the viewer feel what it would be like trying to survive in a world where the peak of civilization is getting further and further in the past (the zombies are mostly just a dangerous part of the environment now, as opposed to being the focus of the story).  Then, something big happens in the fall finale, which drives the story for the remainder of the season, when it returns in the new year.