Nothing, as such. The Raspberry Pi is a great piece of equipment, a fully working computer available for only a few pounds.
The problem with the Raspberry Pi is its suggested use as a teaching tool. The very existence of the Raspberry Pi is an example of one of the most common fallacies in teaching – “I learned something this way, and it worked, so therefore everyone else must learn things this way as well.”
Like most people my age I learned to program in BASIC on very simple machines: Sinclair ZX81s and VIC 20s. What worked for me when learning programming was a simple machine that I could understand fully. But I was interested in programming, and I was willing to put up with the difficulties of a text based interface. Most importantly, those machines were the only computers I had ever seen.
Things are very different today. Children have grown up in a world of computers. They have a very different understanding of what a computer is and what it can do than we did. Getting them to program in Python on a stripped down box merely suggests that such things aren’t possible on a regular PC. It’s far better to get them to download an SDK onto a machine they’re familiar with than to get them started in a completely strange environment.
One last thing: Raspberry Pis aren’t even cheap. The machine itself may be, but you still need a monitor, a keyboard and mouse, and most importantly in a school, somewhere to set them up. And unless you intend to only set them up before each lesson, they are going to sit there doing nothing for most of the time. Of course, you could always put them on a trolley. Putting computer equipment on trolleys has been a great feature of British Education over the past twenty years. The school computers in this country must have covered more miles to less effect than the IT equipment of any other nation on Earth.
If you’re looking for something simple and cheap, the best machine you can get your hands on is on old PC installed with Lubuntu. Many British families probably have an old PC at home that they could use already. What better way to learn to learn about computers?