Input is the reason tablets have not yet replaced PCs. Voice recognition technology is not yet a good replacement for keyboards. It is almost there, but not quite.
Output is going to be more of a problem. Google Glass and competing products are much more likely to be viable since you can see the output. Smart watches are going to need some sort of holographic or projection screen technology to solve the output problem.
These are the same reasons why calculator watches never replaced calculators, even though they were available 30 years ago.