There are a couple simple and some more complex methods to get teh feel. It depends how long you imagien them to be there.1. Fatigue - Change the forced marching time from 8 hrs to 1 hour. After one hour of treking through the hot steamy humid swamp characters must make fort saves or take sub-dual damage (you could be harsh and add armor check penalties as well to that save to show how difficult trekking through with platemail and hevay weapons).
2. Concealment - Give everyone concealment 10% - 50% or so. This gets to be a pain in combat, but it will give an appropriate sense of danger when they are in the thick (imagine dozens of arrows flying at them, not hitting them, ut it will scare there, and little they could do in response, it makes many weak foes more threatening).
3. Water... A little bit of water can wreak havoc on a party, especially if armored. Alligators grabbing and dragging under water is pretty henious.
4. Getting Lost. Give a generic survival check to get lost. Say DC 10-20. Then after traveling for an hour they get a role, use the grenade-like weapons chart to determine where they are. Or some other mechanism. I find having them eventually get back to where they were a few hours ago is the best method of demonstrating the challenge of the swamp. You'd be surprised how soon thereafter party members decide to find some way out of the swamp (spells etc..)
5. Circumstance penalties - These can be used for combat, skills and other roles (even saves and such). Give the native criiters (alligators, snakes, boars, etc..) freedom from such penalties and even pretty lower challenge rating monsters can become a serious threat.
6. I wouldn't worry about diseases, since they are so easy to cure, although, it does reduce the number of spells available to clerics and druids. --But subtle poisons can be effective as well and quicker acting (eg swamp gas = fort dc 13 -1d6 wis) (eg stinging nettles = fort save 8 -1d4 dex) those things can be realing annoying.
These are all negative (or meant to provide challenges). I sould note that rangers and druids are well suited to handle all of these, and it gives them a real chance to shine in a game that they don't always seems so good.
I like the addition of moss and vine covered ruins (I saw these for real in the jungles of Belize where you couldn't walk a 100 yards in the forest without stumbling over a block of stone from some ancient ziggurat). I wil the idea of magical properties and illusions in the swamps (aboleth are my favorite high level monster in swamps).
Any way. I think swamps offer a lot, it is important that the players know about the efects though. You shouldn't hide this stuff, if you do and then say... oh by the way you are all infected with disease.. your players will rebel, but if they know they might get infected it will have the same effect.
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