First the color of the water should be greenish, matching the grass, which should have a very dull green color like the taiga only darker. Possibly make the swamp have water that bubbles up. The trees should be tall and thick, and have dull colored leaves. Since there is a possibility of loop sounds, they should have crickets chirping all day. The night should spawn more mobs in the swamp than anywhere else. Also, the canopy should cover most of the sky, to give more of that "lost feeling" for when you are trapped in a forest. To wrap this up, the swamp should be the most creepy eerie biome (besides the nether) in the game.
If only I could aim.