This kind of thing is usually a hardware limitation -- most keyboards aren't designed to have many keys pressed at once.

On my keyboard, once I have Q pressed I can still press up arrow or right arrow, but if I press up&right at the same time no keypress is registered. This is not specific to Raetikon, I see the same behaviour in eg. Notepad.

There is an exception for certain keys. Shift/Control/Alt are obviously designed to be pressed in conjunction with other keys, so keyboards support these much better. If Raetikon allowed you to rebind grab to a modifier it would certainly help the situation.

Alternately, you can switch to mouse+keyboard control which also gives much more precise movement. Note that the mouse controls listed by the game are inaccurate though, they say:

LMB=Grab
RMB=Dive
MMB=Release

But the actual behaviour is:

LMB=Flap
RMB=Grab
MMB=Dive

It doesn't happen in any other game. e.g. Portal/TF2 I can hold crouch and still move diagonally with two arrow keys.

Is your crouch key in Portal/TF2 the same as Raetikon's grab key? The combinations disallowed by different keyboards can be quite specific.

I'll try remapping that key in tf2 to test that. Good thought.

Yeah, sqweek is right, that's a hardware limmitation we can't do anything against. It's a matter of what key combination you press, that's why it works in Source games but not in Raetikon. You can also rebind the keyboard controls following this guide (http://brokenrul.es/vanilla/discussion/24/controls-and-special-keys#Item_7), try out a few things.
BUT I would definitly recomment the mouse + keyboard controls to you!

v01pe: was my comment regarding the mouse buttons being incorrectly displayed in the controls menu prominent enough or should i make a new topic for that bug?

Originally posted by sqweek:

v01pe: was my comment regarding the mouse buttons being incorrectly displayed in the controls menu prominent enough or should i make a new topic for that bug?

No need for that, I already fixed that, will be rolled out with the next update! Thanks for the report!