The software side is a bigger barrier than the hardware really - current high end smartphones and tablets have the processing power, etc. to handle even most desktop web browsing, productivity, etc. stuff these days - even something like visual studio isn't a far cry - I can hook my tablet or phone upto a monitor or TV via HDMI/MHL and a keyboard and mouse via bluetooth (only due to connectivity limitations) and use it not that unlike a desktop as far as it goes other than the OS isn't really developed with it in mind.
Even for gaming its not that far off - and converging faster than PC tech is moving on.
Even for gaming its not that far off - and converging faster than PC tech is moving on.