What do you mean? Was Thor different back in the day?
For a long time, Thor's powers have really just been super-strength/endurance/durability and the ability to wield Mjolnir. He couldn't even fly, he'd just toss the hammer and let it carry him where he needed to go. Other than that, his powers have fluctuated wildly over the years: sometimes he can summon lightning, other times he can't, sometimes the hammer can shoot lightning, other times it can't, his durability will let him tank space battleship cannons to the face, he'll have a hard time standing up to Ultron, it's all over the place. Probably another reason why people don't consider him as much of a Superman analog, Supes has always had a pretty constant skillset, though the limits of those powers are all over the place.
Doesn't this describe like every instance of the writing going south? There are very few instances of "bad writing" that don't come off as the writer simply being mean spirited rather than a lack of skill.
It varies. Most of the time it's just a writer having a good idea that doesn't pan out, not being able to stick the landing, the artist not being suited to the idea or, more often these days, plans being changed because of a crossover, event or editorial dictate. I can really only think of a few writers I'd consider 'bad' by any metric, most writers either aren't to my tastes or they just do nothing for me. Former DC editor-in-chief, Dan DiDio famously hated scads of characters and wanted them killed off, turned into villains or just plain written out of existence. He's one of the writers I'd say is just plain bad, but he was also infamously mean-spirited towards the characters he didn't like.
Spill the tea.
I don't actually read Superman, so what I know about his run is mostly second-hand, but people went in expecting huge, epic stories with stuff happening in his first issue that would receive a payoff years down the line, the kind of long-form storytelling he excels at and is loved for. That wasn't quite what happened. There's a lot of small nitpicks, but there's also a couple of big things that really rubbed fans the wrong way. First off, he tried to split up Lois and Clark. You don't have to be a fan of the characters to know that's a
huge no-no. They got back together, but that killed off a lot of goodwill he may have had with fans. The other thing is Jon Kent, Superman's 10 year-old son. He's best-friends with Damien, Batman's son and since they were both about the same age, they had frequent adventures together. These were genuinely fun and heartwarming stories - Damien's an edgy try-hard pre-teen who takes after his father, Jon's an over-excited puppy with superpowers who takes after his dad, the comedy just writes itself. This stuff was almost universally beloved and people were begging DC for an ongoing series featuring the kids. First thing Bendis does when he takes over? Ship Jon off to an alternate dimension where time goes faster and ages him up to be about 18. No one liked that and thought it was a waste of a character. And then there's Bendis' 'quirks' of writing dialogue - google 'Bendis speak' for many, many, many such examples. You either love it or you hate it and a lot of people really don't like it at all. It's like Quentin Tarantino's dialogue and monologues, love it or hate it, it's a fundamental core of his style and a big chunk of his appeal.
