I would have finished STALKER long ago if it wasn't for all of it's anoying glitches and lame mission system.
It isn't really fun to hike miles to defend a position then hike all the way to the trader for your crap reward. Then when you finally get back for the story missions, the trader askes you for for another mission that you've already done. And you've got a time limit to do it in. Rinse and repeat.