Most keyboards only allow you to press a few keys at once. The problem is especially obvious with the arrow keys in videogames, even more so if you have two players on the same keyboard.
Wikipedia calls it rollover. Unfortunately, I've never seen this listed in the specs for a keyboard. Most keyboards have 3-key rollover, with a maximum of 6 for certain combinations. Not good enough. The old Model M used to have full n-key rollover.
Now, where can I find a keyboard with high rollover?