I'm a big fan of some of the changes made in 4e regarding conditions. Being paralyzed and stunned means waiting until it's your turn again, and maybe you can take an action if it wears off, or maybe not. Not being able to take an action isn't fun, nor is having to roll on a confusion table to see what action you'll take. Nor is ability damage fun: who wants to recalculate attack and damage every time a ghost hits you?
Far better to limit certain action choices, i.e. instead of having an effect cause -6 to strength, better to simply have the character deal half damage. Instead of stunning a character, have him fall prone --it doesn't prevent the player from making choices, but it does limit them.
Below are the common negative conditions (I omit Deafness and other tedious conditions) as I have been using them in my present game. I am particularly proud of how Diseased and Delirious work.
Afraid: You suffer a -2 penalty to your attack rolls and spell DC for the remainder of the encounter. Additionally, you cannot benefit from leadership bonuses or morale bonuses (such as those granted by clerical spells). Characters that are immune to fear cannot be made afraid.
Blind: -5 penalty to attack rolls and spell DC. Cannot target ranged attacks or spells to farther away than ten feet.
Damaged: Some monster attacks disable magic items semi-permanently, but do not destroy them. These items may be repaired via arcane rituals, but until then they offer no benefit beyond a non-magical item, and cannot be used as normal.
Delirious: You cannot cast spells of the highest level that you are capable of casting, nor activate magic items other than potions. Nor may you cast spells that are affected by metamagic feats. Nor may you use per encounter maneuvers such as those gained by the warrior and rogue classes.
Diseased: Effects that restore lost hit points and grant temporary hit points have their effectiveness reduced by half (round down).
Prone: You suffer a -4 to your AC. Prone creatures have their speed reduced to 1. You may stand up as a move action.
Slow: Your speed is reduced to 2.
Surprised: You suffer -3 to your AC and saving throws. You are normally only surprised during the first round of combat before you have taken an action.
Weak: Your attacks and spells inflict half damage (round down).
I want to have more negative conditions, but I'm struggling to come up with equally elegant mechanics. Some sort of "Rotting" condition might work, as might "Exhausted" or "Dehydrated". The drawbacks need to be significant, but not a pain in the ass to remember or deal with. I use poison as a damage type, so that's not really a venue for ideas.
All of the conditions listed above are non-trivial, and aside from prone require magic or time to end their effect. Some negative conditions that have non-trivial effects but are easily dismissed might be good, such as a Dehydrated condition that could be ended by drinking any magical potion.
I dunno, I'm still working on it.