The Raspberry Pi is designed to be an embeddable device. So long as the environment around the RPi isn't exceptionally warm and humid, the RPi will be fine in an enclosed container. You will never get an air tight seal with a lego case anyway. I mean, you have to leave holes in it for the ports after all.
Since LEGO bricks are made of a nonconductive plastic, you don't need to worry about them not touching things on the board. Nothing is hot enough to melt them (unless something is very, very wrong) and plastic is non conductive.
It's been pointed out that there may be some issues if the LEGOs are putting pressure on a chip. I had assumed that it was obvious that strong forces shouldn't be directed to the circuit board, but for clarity I'll add the warning: do not put pressure directly on circuitry.