Yeah, playing with the Oblivion Lost mod can bring back some of the original fun, because it unchains all of the enemies' AI and adds all of the enemies that had been removed, some of which can really freak you out if you don't know what they are.
This led to two of the most terrifying experiences in my gaming history. Real fear, too. The kind of fear that makes you drop what you're doing and get away from the situation as fast as you can.
Now that I think of it, it was just one. The second was only startling, and I laughed about it afterward.
The first one was in the Cordon. I had only been playing for an hour or so and was still using an MP5(I believe it's called a Viper in vanilla STALKER). I was driving my sweet ride(Yeah, Oblivion Lost puts cars back in the game, as well as blowouts, faction combat, mask effects [Wearing masks puts an overlay on the screen that looks like you're wearing goggles or whatever, and you can see condensation on them], and a bunch of other things that should've been in the game on release) down the main road and was a few yards past the farm area when I saw the body of a rookie stalker on the ground 30 feet or so off the road. I stopped, got out my trusty gun, and got ready to see he had anything useful. I was creeping up to it pretty slowly because I still hadn't seen any dogs around, and wasn't sure where they were hiding. At about the halfway mark I hear it: That low, ragged panting that signals invisible death for anyone who isn't seriously hardcore. I still can't see the Bloodsucker, and I'm not seriously hardcore, so I panic, turn around, and sprint to the jeep. I managed to get in and floor it, but they were right behind me. I suddenly hit a bump and rolled the jeep, landing it upside down with two bloodsuckers about ten feet away. I don't remember what happened next, I'm guessing my mind blocked it out.
The other thing was in the first underground area, where you see your first Bloodsucker in Vanilla. I had gotten to the part where you start seeing lots of military guys, but it was completely empty. No Bloodsucker, no military. I keep going forward and find the first military guy, on the ground, with most of his blood on the walls. Deeper in was more of the same, there was a whole mess of Bloodsuckers in there and they had eviscerated the military guys. In what would have made an awesome gif, I looked down a hallway and saw a bloodsucker run by exactly like bigfoot in that famous video. Low to the ground, arms swinging, everything. It made me jump, but afterward I laughed at it.
Seriously though, that Oblivion Lost mod rules.