I've thought a long time about damage types and how much they add or detract from game play. What I've come up with is most of the time they really can be ignored...however, it does added significantly to my playing experience when I think about what the effect of shooting an arrow at a skeleton, and even though my bow is my best attack and most damaging weapon, I think about what the effect in "non-game" terms and decide that asking the cleric to borrow their mace is a much more logical attack, since they are going to be turning for the first few rounds.
This is just one of the things that were lost (for the most part) when I switched from D&D 3.5 to 4E, but is seemed minor when I was playing 4E a lot, and only after playing Pathfinder at a Con did I realize what removing the damage types did to the basic combat, and after battling a few creatures where there was a real bonus/penalty of choosing the right or wrong weapon did I remember what this added to the game.
To me switching things up in a way that makes logical sense is much more enjoyable. I don't need everything spelled out as a GM either, if a clever player uses a vial of acid against a shield guardian in an effort to do more damage rather than trying their typical feint and sneak attack move that will deal a lot of damage, then as a GM I have to say, "Do I want to encourage this behavior or simply force them to do the 'tried and true massive damage approach'?"...So, for the sake of encouraging this behavior, may I give them a significant damage bonus (and maybe I up the creatures HPs at the same time), however the player just sees the damage dice they just rolled, and then next round seeks to do something equally clever...tell me it doesn't beat the crap out of hearing the exact same battle plan round after round, or simply hearing "I attack that one...<roll>...does an X hit?".
Depending on the party, it may get a little tiring of having players "beg for bonuses" every round out of jealously seeing the extra damage their companion just received but I still think it beats the alternative, and you can also apply it in a negative aspect...by first making the hero commit to the attack before announcing what bonuses/penalties they receive and when they try to weasel out after finding about a penalty, simply saying "It's too late the attack has already been made."
What I've done is pick some of my favorite types of damage, I admit I got a little crazy, but honestly most of these will be seldom seen any way, and in many battles admittedly it won't matter that it's a slashing weapon verses an opponent in ring-mail but if the player actually makes the effort to react to something, even if its not in the rules or it's not something that I'd thought about, what the harm in rewarding them with a dice? The still need to get a good roll on it or the extra dice didn't matter in the result, but it did matter in their mind.
The last group is Righteous damage, which is where radiant damage is, but I also have Natural weapons in here, my thinking here is Natural weapons (claws, bite and fists) are a form of "courage" attack. This rationale came to me when I wanted a hero to have a courage based attack form, and to me nothing would be more courageous than to leave your weapons behind and simply attack with ones hands or body. Some archetypes (thinking warlords or cavalier here) might even have auras of courage that would make their weapon damage act Righteous damage in addition to its base Physical type. The paladin or cleric archetype would already have abilities that would grant them radiant damage so they would have the ability to affect creatures such as spectres or other incorporeal creatures.
"Damage Type" as its core, I don't see as complicating combat. Rather, I see it turning combat into a mental exercise where players rather then simply saying "I attack!", will rather review their heroes arsenal and select the best weapon that they have verses the target they are attacking. Sometimes this choice will be selecting the attack with the largest attack surface, other times it will be more of a ray or other more controlled attack pattern. Sometimes the choice is, how much energy to expend in a next round to hopefully end it sooner, maybe its to inflict less damage to more creatures or maximum damage to one, is so then what type of damage works best.
So, there is always going to be the "standard attack" that the hero's will fall into, but as a GM there are environmental things that can be done to give the creative players a few one shot damage weapons for free in room...A cask of oil by the desk, a glowing brazier on either side of the altar, chandelier hanging in the center of the room, and curtains covering a few entrances...depending on the opponent many of these could be used as a more effective weapon than simply swinging an sword or axe.
The creative rewards don't have to be directly related to actually killing the enemy. Often, the party will out number the enemy when fighting a deadly creature, if one or two of them can sacrifice their actions to cause the creature to not attack, or to not use its most dangerous attack, then this will give the rest of the them a free round to deal damage. When they are out numbers by hordes or swarms of weaker creatures, then maybe simply causing the creatures a round of confusion or fear with a creative attack is better than taking even a small group of them out.
Ultimately, I find combat a delicate balance of fear and confidence, where there's just the right amount of role-playing and war-gaming going on at the table. This is often a challenge due to the styles of the various players that are assembled at the table, but I think putting together scenarios that allow of environmental advantages or weapons, as well as clues that may allow for a short circuit of the melee battle by dealing with an environmental puzzle. Not every battle requires a puzzle be solved or optimization to be sought after by the party, sometimes the best answer is to let the barbarian do what he does, or the rogue do their thing...but in cases where the battle is going to be more difficult it probably will pay to give the party an "out" or at least a few things to try...just in case there's a TPK, I find it goes over better in the "WTF happen" post mordem talks to say, "Well, what you guys didn't do is _____ or _____". This maybe just me, but I don't like killing a player when they were just executing on a good idea, even when their dice are out to get them, but sometimes the heroic thing to do is to let the hero die being heroic.
Here is my current working list:
Slashing Bladed weapons that attack with cleaving or slashing motions, also includes teeth and claws that have tearing attacks
Blunt Most rounded, rough or dull pointed weapons, fists or stomping attacks
Piercing Pointed attacks such as arrows, or long stabbing weapons that are primarily designed to pierce with a point
Fire Heat, burning or otherwise hot or actual open flames
Lightning Electric, Lightning or any type of voltage energy charge
Acid Organic material dissolving liquid or gas attacks
Cold Cold, ice or frost based damaging attacks
Explosive (Force)Non-physical explosive trama attacks, such as arcane missiles, sonic or thunder attacks
Dissolving Inorganic material dissolving liquid or gas attacks, corrosive attacks
Vile (P/D/E/N/T) - often has long term lingering effects
Poison Attacks that drain points of attributes
Disease Turning a creatures own immune system against itself or simply over powering it
Energy Drain Attacks that drain life force, stamina or mana out of a creature (or possibly item)
Necrotic These attacks reduce the maximum health of the target
Taint A measurement of supernatural corruption, this is somewhat like damage to ones soul
Radiant Holy/Divine energy attacks
Belief Similar to radiant, but lacking in divine origin. Having extreme hope, courage or faith in oneself is enough
Natural/Living The weapon itself is actual alive, a fist, claw, bite or grapple are simple versions of "living" attacks (extension of Belief)
(Healing) Recovery of health (not really a type of damage, but more anti-damage)
(Craft/Repair) Recovery of structure of an object (not really a type of damage)
Sanity Mental degradation that event leads to the inability to believe ones senses or makes one incapable of personal actions
Illusion Mentally convincing a creature that is has taken damage, which actually makes the damage real
Psionic This is actually physical damage inflicted by a psionic attack