Turning off particles will increase your performance, and your ability to see while firing weapons like the Sidewinder, but will also eliminate the ability to backtrack a smoketrail to find an ambusher.
Tells DK whether or not to render the cool, atmospheric, creepy volumetric fog that was added to the engine's feature set.
Toggles the rendering of dynamic lights (lights created by moving objects, powerups, etc) on or off. Dynamic lights can be a pretty significant hit to performance if you have a mid to low end CPU and video card combination, but on most modern cards and systems you can afford to leave them on and gain the significant "oomph!" they add to the graphical splendor of a heated firefight.
Turning on gl_flashblend removes standard dynamic-lighting and instead creates a "glow in the sky" that moves around with the object that's creating the glow, instead of just lighting up the architecture around it. I have no idea why this function exists. Interesting to play around with, but kinda ugly.
This helps to control the darkness of shadowed areas. Valid parameters range from 1 to 3; you can set it to higher than 3, but you'll not notice a change. At 1, you can see that your weapon is darkly shaded, and the items are shaded too. The items will actually pulse brighter regularly. At 2, you can see that the gun and ION Pack are both much brighter; this is approxiately the brightness that items will pulse to at 1, and at 2 they no longer pulse.
World-texture level of detail. 0 is the sharpest, clearest, highest-quality textures, and uses the most texture memory, thus adversely affecting performance. 3 is the blurriest, murkiest, lowest-quality textures, but also uses less texture memory and can increase performance.