No 5. Keyboard
Soft keyboard is a real productivity killer. Soft keyboard on smartphones is OK; neither you use your mobile phone for photo editing, video editing or other heavy duty works nor is it meant for such work.
Whereas, a tablet is really an in-between device; neither can do real work with it nor use it as a traditional communication device.
No 4. Portability
No 3. Software
There are over 250,000 apps for the Android and over 425,000 apps available for iOS. I guess there is a reason these are called ‘apps’ and not ‘applications’; they are no match for desktop’s power, they come nowhere close to the creativity and freedom offered by more than 30 years of evolution of PC applications. And there is no count of the millions of software available for the desktop.