Dmitryi & Elf:
The default keyboard driver doesn't allow multiple simultaneous key
presses. This is a big problem for an FPS, as it means the player can
either strafe, turn, or shoot. Looks like only four or so keys can be
pressed simultaneously (tested with a couple Quake clients, Tyr Quake
and
Tenebrae). This might be an X limitation (?)
On my system, pressing multiple keys at once (for example in Quake3
Arena, which doesn't run properly for other reasons here) works fine.
You might want to check your settings in xorg.conf and your desktop
environment's keyboard and/or accessability settings.