Since touch screens are all the rage today, I thought I would expand on what I wrote last month when discussing stylus input. In that piece I explained the difference between resistive and capacitive touch screens - generally resistive screens are less expensive, require more pressure and are increasingly being replaced by capacitive models that are super sensitive to your touch.
If you compare touch devices, you will begin to notice more differences. Again there is a low cost approach that works like an ATM - touch an icon and an action is triggered. More sophisticated screens incorporate three additional capabilities - multitouch, physics and gestures.
As the name implies multitouch devices let you use more than one finger at a time - tapping with two fingers means something different than tapping with one - much like a right click on a mouse produces a different result than a left click.
A physics capability means the response of the screen mimics physical actions. For example, swiping your finger across an ebook page causes the page to appear to "turn" like a real book. I first noticed the realism this adds one time while reading on my iPad and had the urge to actually pick up the page to turn it. it became that "real" because of the way the device responded to my movement.
Finally, gesture means the screen responds to movements - such as the swipe across the screen mentioned above. A common example is that opening or closing two fingers changes the magnification of the displayed image - combining multitouch (two fingers) with gestures (finger movement).
Both e-readers and tablets will drop in price sooner rather than later. We've already seen a significant drop in e-reader prices as a result of the iPad introduction. One way to reduce prices when the pressure becomes intense will be to use cheaper, less sophisticated touch screens. There's nothing inherently wrong with this but you need to make sure you know what you are getting before you buy.
Without gestures, tapping an icon would turn a page. Without physics pages would turn in some programmed fashion. These may seem like minor issues until you experiment with different products. In general the lower priced gadgets will have the least number of these capabilities. How "natural" it feels to use a touch screen will depend significantly on if and how multitouch, physics and gestures are incorporated.