I've been doing my best to look at this from all sides for a while. That's why I took so long to start replying to this thread. This is the best I can come up with.
There are a number of ways that the writing structure can make powers feel similar to each other. The fact that every power is written up in exactly the same format, and that that format is very technical. The fact that in 4th every attack has the attacker roll when 3rd and 5th both have some abilities that the defender rolls instead. The fact that most classes follow the same structure for when they get a power, and share the same mechanic for recharging them. That all adds up to a lot of superficial sameyness that I can see leaving an impression on people. At the same time if you really look into what goes on at the table, there are tons of differences to how characters from even the same class operate. People have come up with tons of examples to explain this. Objectively, 4th has at least as much differentiation as 3rd and 5th, they're just in different places.
What we're really discussing here is emotions and expectations, and those are very important. I read a post from a designer a while ago discussing what advantage system he'd go with for his upcoming project. He started out with just a flat bonus system like 3rd, but then began to experiment with the roll two dice take the highest like 5th. The math for both is pretty similar, so it all came down to how his testers felt about them. All of them liked the roll two take highest version. There's something to rolling a bunch of dice that just feels good. Nothing about mechanics there, just emotions.
Which just leaves us circling around the same arguments about terminology. We're really just splitting hairs about where the differences are, or what the definitions of our terminology means. In the end there isn't really a way to win this argument.
The last thing I'll say on this matter is why 4th fans feel that this is an important discussion. It's the fact that the saying "all the powers in 4th are the same" is so frequently used when talking about it. The reality is that the problems with 4th are much more nuanced than that. There are some people that might only hear these quick, meme-y criticisms. As fans of 4th, we'd just like the criticisms of the edition to be more complex than that, so that 4th can have a legacy instead of just being brushed off.