I've written about this a bit before when describing the benefits of the iPhone, but I think what many people are missing in their analysis of the coming tablet computers - summarized as 'who needs one?' - is they're forgetting the benefits of combining the user interface with the human interaction device. In other words, touch changes everything.
Apple wasn't the first to do touch by any stretch - Nintendo launched the DS years before with it's secondary touch screen as an integral part of the device - but I think many of us didn't realize how revolutionary touch interfaces are until after the Cambrian level explosion of apps that came when Apple opened up their platform. Getting rid of the mouse and keys, and letting users interact *directly* with what they're looking at on a computer screen opens a whole new level of design, flexibility and ease of use that we haven't seen before in computers.
In other words, no more mental associations need to be made between hardware and software on the part of the user. You don't move this thing over here (keyboard, mouse, joystick, buttons, etc.), to affect a change on the screen over there. You directly touch the thing you are working with. This is a very big deal.
I have to admit, I first thought Nintendo's touch screen was a gimmick. And honestly at the time (2004), I was sick of using a stylus on various devices and thought it was just the wrong direction to go. But then after you first play a game like Nintendogs where you 'throw' a Frisbee by flinging it across the screen, and 'pet' your dog by rubbing its head, you quickly realize that there's a real visceral sensation you get from that way of interacting with a game that is completely removed if you have to use something like a joystick or mouse to do the same thing. Trust me, I've played the game on a PC using an emulator where your mouse can take the place of the stylus, but it just isn't the same at all.
I honestly don't know if Apple realized the importance of touch at first - I think they were looking for the simplest/cleanest UI solution and decided on a touch screen as it was the most flexible in terms of creating and iterating an interface. Besides the basic multi-touch gestures, there really wasn't much in the original launched apps beyond the WIMP style interaction we're used to on a PC.
No, it was the thousands of app designers out there that have really embraced the touch paradigm that pushed the limits on what could be done, blowing all of our minds. The first app that did this for me was BeatMaker. It had dials, switches, multiple screens, tap-pad areas for creating your own rhythms, and more. The results are studio-quality tracks from 'a phone'. WOW! That's when I realized what was happening. It wasn't that the iPhone was some special magical device that had special technical abilities that allowed this. That sort of app could have been made before - but by forcing developers to *only* use touch, rather than mapped buttons, etc., there resulted in an burst of innovation.
Now here's the paradox of touch - there are certain killer apps that it just isn't good for. Email and Tetris to name just two. One one level this makes sense - these apps were invented long before touch interactivity was even considered, and thus optimized for their hardware counterparts. People much prefer using a Blackberry to do their mobile email and I can't imagine typing this post out without an actual physical keyboard. And I'm pretty good at Tetris, but there's no way that I could ever get the same scores I can on a Nintendo DS while playing the iPhone version - no matter how much I practiced. Some activities just need physical buttons.
That said, it's quite apparent that just about *every other* app out there can benefit greatly from the pliability of touch interfaces. Especially for those not quite familiar with the device, or for apps that require very specialized interfaces such as most games. Newbies benefit from being able to engage directly with whatever on-screen widget it is they're trying to manipulate - without having to also deal with learning about some piece of hardware. Games benefit from being able to be as creative as they like, and mapping controls to anywhere around the screen.
So this brings me to Tablets, which are obviously going to be the new hotness this year. I've been big on web tablets since I first got my Nokia 770 years and years ago. Having a convenient little touch screen device which lets you do your basic web activities without having to be at a desk is awesome. On your couch, bed or toilet at home, and in the back seat, train or plane while traveling, or under the table at a boring meeting is just some of the places where their form factor and functionality fits perfectly.
Here's the thing though, I don't think tablets are going to be the Next Big Thing until they finally take true advantage of their touch capability. Right now, most tablets that have been launched use the same sort of WIMP interfaces and paradigms we see on a desktop. This just really doesn't do it. This is why, to me, I'm more excited about Android-powered tablets or whatever it is Apple comes up with, than I am about Windows/Mac or Ubuntu powered tablet based PCs. The paradigm has to change - your finger is not a mouse, and no keyboard means that data entry is a pain as well. I don't want to use any sort of stylus either - I stopped using a pen regularly for work back in the early 90s, and I don't want to start again. (This means that Windows tablets really need to be rethought).
What's interesting is that I have a feeling the two revolutions are going to bump up against each other before too long, because many websites are just a pain to use with your finger - you end up having to zoom in to click some tiny icon, which then launches some Ajaxy pop-up which you don't see because you're zoomed in, etc. This will only get worse with bigger/higher resolution screens. It should be interesting to see if there's a push for "touch-friendly" websites, similar to the way there was once a push for mobile-friendly sites, or whether browsers/OS developers will find a more general solution.
Anyways, no matter what, I'm looking forward to the tablet revolution! :-)
P.S. Personally, I wish everyone would just call tablets "Netpads", since I own the netpad.me domain, and then it'd be worth something... but I guess the "tablet" nomenclature is sticking, so I'll use that. :-)