Apple played their cards well by pre-empting the market and releasing iPad ahead of the competition. This has set the benchmark remarkably high for other vendors. Although tablet's have been available for a long time, they were never a compelling competitor to a laptop in features or cost.

Netbooks tried to fill the gap, but without any dramatic improvement in battery life, weight, or functionality. iPad did that.

What's missing in iPad -
  1. iPhone 4's design
  2. Gyroscope
  3. Face-Time
  4. Dual-Core Application Processor
  5. MicroUSB (w/wo OTG)
Chrome OS
This is a clear bet on attacking the weak points of other modern OS'
  1. Expensive - Win-7 Basic is usually $20-$70 in developing countries.
  2. Broken - IE, AntiVirus, Auto Updates
  3. Complex - 30 minutes to setup, and technical help for support
The other bet is on 'Thin' client. A balanced machine in price/ performance/ efficiency for increasing usage of internet. This area has been tried several times in vain, by Sun Microsystems and others like Novatium (here in India).

They are an outside entrant. They have a more flexible approach, applicable to multiple devices - App Phones, Tablets, Thin clients etc. I am not aware of any disruptive design goals, but rather to have maximum compatibility with different devices. This approach depends on having compelling products to compete with ChromeOS, iOS, Android, WP7, and even Windows 7.

Finally, the fourth contender comes again from Google and it's army of Mobile partners - nVidia, Samsung etc. This is a head-on competitor to iPad. Till now their differentiator seems to be on 3D graphics accelerated interface. That added with other aspects which differentiates Android against iOS are on the table. 
  1. Multiple versions vs. One iPad
  2. Multiple Open interfaces (MicroUSB, replaceable Battery etc.)
  3. Better Google Apps support (Free Navigation etc.)
  4. Open and Free development platform (Apple charges $99 per year)
The space is getting serious traction and its difficult to take sides 
  1. Apple - iPad - iOS,
  2. Google - ChromeOS - CR48, Android Honeycomb with nVidia
  3. Microsoft - Netbooks with Windows 7
  4. Samsung - Galaxy Tab - Froyo
  5. Nokia - Meego (+Intel) No product
  6. RIM - QNX OS - Playbook
  7. Misc - Novatium Neon, OLPC, NotionInk's Adam, etc.
I am yet to take sides on this one. I wanted ChromeOS, but they'll debut with Intel Atom rather than ARM Cortex A8/9. Also it'll have its own first generation issues like Cupcake, Donut.

Which side of the Tablet fence are you rooting for?