What's a "serious document"?
A longform essay. A term paper. You know, the stuff you actually want a word processor for.
And until strong multi-tasking is added, you are unable to grab articles and data from online and look at it while incorporating it into what you are writing.
That's serious writing.
It works for emails. It works for little notes you might like to make. It doesn't work for creating in depth business proposals, term papers, etc.
Even on emails it can be somewhat lacking if you wish to multi-task while writing the email. First close out your web browser then do your email.
It's a gizmo, not a serious tool. Touch typing will be difficult, if near impossible on the thing. I think it could work well for showing pre-prepared PPT presentations if you plug it into a projector, but creating a complex presentation might not be easy on it. And space is limited. If you load it with media, etc then you crowd it even further.
The tweener 'puter is already here - it is the Netbook. Around 70 million PLUS netbooks will be sold this year. The estimates are for them to go over 100 million fairly soon, possibly next year. These are slightly stripped down, fully functional PCs that actually have a copy of Windows on them.
Had the iPad been loaded with something closer to a full copy of OS X then maybe things would be different. Had they gone with Intel and made it compatible with Mac apps like Netbooks are compatible with PC apps (and the HP Slate will be as well) then maybe it would be different.
Realize that within about a four year period that Netbooks will have sold better than the iPod has sold in its first NINE years. That's massive growth and that's ultimately why Apple is coming up with the iPad. They don't want to copy others with the Netbook. I think that has a lot to do with Jobs' ego on the matter. He thinks Apple innovates. It doesn't. Apple didn't invent the MP3 player. They made the MP3 player practical. They gave it an interface.
But the MP3 player really just inserted itself into a market that already existed. The iPod didn't create a new market, it just pushed an old market in a different direction. I don't see the iPad as doing the same.
Remember, for all the accolades of the iPhone, it accounts for around 2-2.5% of the entire cell phone market. It's very profitable for Apple and good for them, but it's far from being the iPod of the cell phone market.
I'm one who thinks that Apple has gotten lazy. They know there are a bunch of people who drink their Kool-Aid and will fork over money to them no matter what. They could scoop a couple of floaters out of the nearest toilet, fire them in the kiln, coat them with shiny white or shiny black paint, and sell them on the market as the iTurd for $400 and a bunch of people (many hear) would get excited about it and make excuses for why it is so wonderful.
The iPad is a neat gadget. I consider it a waste of money, but Apple will make money off of it. There's a sucker born every minute and Apple is well aware. And that's what makes them brilliant. They know how to get you to pay out big bucks to them so they can get their awesome 19% profit margin. That's why I own Apple stock, though I don't buy Apple products any more. I don't like getting ripped off, but I don't mind making money off the suckers getting ripped off.
But the iPad will see limited success. People will tout six million in sales as a huge success and will likely try not to compare the number to netbook sales overall. And it'll be great for the stock price, but the challenge will be expanding beyond a core or beyond large scale sales to school systems, etc that want the iPad for some kind of dedicated usage.