The girl next to me on the bus had an iPad, so I asked her how she liked it. I expected the usual "it's great", but instead I got "I don't" I asked her why and she said there was too much stuff it couldn't do. I didn't get a chance to dig deeper.
Someone else I know bought an iPad and immediately got someone to jailbreak it for them. Why? Because there's too much stuff it can't do otherwise. When I asked for examples they couldn't really give me any. This is a non-technical person, it's not that they wanted to do anything special.
Obviously, there are things that an iPad is not ideal for. And there are things that an iPad can't do. But there are a huge number of apps and for most people, I just can't see what it "can't" do. All most people do is email and Internet anyway.
It can't run big programs like Photoshop, but most people don't need that. You can't run Microsoft Office, but you can get word processing and spreadsheets.
The touchscreen keyboard is not great for typing a lot, but most people don't type a lot. And if you really need it you can use a Bluetooth keyboard. Most people are happy typing on their cellphone! (I manage to type quite long blog posts only iPhone.)
I would guess one of the reasons for this idea is that people get overinflated expectations from all the hype. And when it doesn't (can't) live up to them, they have to blame it on something.
Which is where the critics come in. People don't remember the specific criticisms like the lack of multi-tasking or cameras or memory cards. All they remember is that "there's stuff it can't do".
And there's always the most common reason people say you can't do something - simply because they don't know how to do it. We get that all the time with our software. You ask people how they like the software and they say it's ok but they really wish it could do xyz. Half the time it already can, they just didn't know it.