How vulnerable is the Raspberry Pi without a case? I've seen the many Lego cases which look great, but I'm constantly wondering what the risk of such a case is. Will a Lego case successfully protect the Pi just like a regular case will? And what is it that a regular case protects the Pi from?
The pins, headers and external components look fragile and easily bent.
It looks easy to accidentally short GPIO pins and other conductors on the Pi by (e.g.) dropping it on a coductive surface, brushing a conductive object against it or dropping a conductive object onto it while it is powered up.
a number of the sockets, the HDMI connector in particular, are quite stiff; anything that makes it easier to grip the board edges or otherwise aid plugging/unplugging cables without exposing fragile connectors to undue forces therefore helps
Even something like the punnet goes a long way to solving those concerns. However, if you do put it in a case:
connector joins stick out of the bottom of the device, as does the SD card socket, so simply resting it on a flat case bottom could cause problems - there may be mechanical forces on the solder joins when inserting/removing cables, and it may be difficult to insert/remove SD cards or lay the device flat when an SD card is inserted
there isn't a great deal of clear space on the bottom of the device where feet / pads for it to rest on can go, so these would have to be very carefully positioned if such a solution is taken
I reckon the best case is The Punnet; it is a case made from card. You can print it yourself, fold and glue, and you will have a case in minutes. It protects the RPi from dust, which is the main threat.
Other cases may protect the RPi from light spills or unwanted electrical interference, but I don't think it is worth it given the cost of the RPi!