I will start off with the Power of Pain and move right on through to poisoned weapons and Dark Lance Technology. After this, part 2 will be moving on to tactics and unit combinations that are harder to dig up, some of those hidden gems.
Power through Pain:
What this does is give us the first army that truly gets stronger as the game goes on, even as you lose units. Those that remain have the opportunity to gain in durability and strength. What this can do on the tabletop is to very quickly turn the tide of the battle. Dark Eldar are still a paper thin army, however a couple pain tokens on a few units, you suddenly can feel unstoppable to your opponent. Twice already I have had friends call me up with "I couldn't get through the feel no pain". Add in cover saves, reavers going flat out, incubi warsuits, and units with T5 or pain engines with T7 3+ saves, and opponents are quickly facing their worst fears. If you don't have the codex, the first pain token gives you Feel No Pain, second gives you Furious Charge, and the third makes you fearless. You gain pain tokens simply by wiping out an enemy non-vehicle unit through shooting or close combat.
What this means is that against armies with smaller units you can literally power up faster. You will need to decide who gets the killing attack when it comes to shooting, so plan out your shooting order like the obsessive Archon your are. Because of this Power through Pain, you will want to eliminate units to the man. Don't leave anyone alive.
Another cool thing is that pain tokens are generated immediately. This means that units with multiple initaitives can generate a pain token during a close combat round. For example, your archon is running around joined up with your incubi, and they assault two units in a single multi-unit assault. The first unit only has two marines, that your archon quickly kills on initaitive 7. Your incubi then take down most of the other marines leaving 1 or 2 of a 10 man squad. When the marines attack back, your unit now has the Feel No Pain special rule, because they have generated a pain token on a previous initiative. Hows that for underhanded and nasty. You don't think this happens that often? I've accomplished this in the last two out of three games.
Sharing the Pain
This one is pretty simple. When an independent character joins a unit, they share the pain tokens. When they separate, you divide up the pain tokens equally, with any left overs going where you decide for them to go. This simply means that when an Archon that decides to start the game joined with a group of Wracks standing on the ground leaves, he can take that pain token with him. He can join a unit of Incubi mounted up in that raider, and zoom across the field. Both the incubi and archon would now carry that pain token and get Feel No Pain.
The Same thing applies if you are running Haemonculi. He starts off joined to that unit of Reavers. he leaves during the movement phase leaving the pain token to the jetbikes, and the reavers then turbo boost across the field in order to get their 3+ cover save. Along the way of going 36" they go over that group of long fangs and deliver 2d6 S6 hits and 4d3 S3 hits. Not only are you getting your 3+ cover save, but you now also have Feel No Pain to help protect you from weapons that remove cover saves. There are all sorts of ways to play this, including doubling up on the pain tokens. This tactic is very very flexible.
Poisoned Weapons
We get 36" heavy 6 poisoned shooting weapons? So what, they take a 4+ to wound. Lol. These things are nothing short of phenomenal. Venoms running around taking 12 shots, moving 12" shooting 36". Wow. Scourges, Trueborn, Warriors, Hellions and others take serious advantage of this. 36". Most of the time we are fighting vs marines, and amazingly enough they like to get in cover. No matter with our splinter cannons, we see one of them, even a shoulder pad, we shoot. Pelt them down with lots of wounds and let that 5 man unit fail their saves. The same goes for monstrous creatures and another other units trying to get across the board. Long range with lots of shots, our poisoned weapons don't really care if your hiding from them. They are only AP5, so pelting marines with 40+ poisoned shots is going to hurt and they can stay in cover the rest of their short lives (very short).
Last game I played I had two units of 20 warriors in the backfield with 2 splinter cannons each and 2 venoms running around. It was literally 48 splinter cannon poisoned weapon shots a round the entire game (til the last round I think when I lost both venoms). You can generate a lot more than 48 shots. In fact a single unit of scourges can put out 42 poisoned shots a round standing still or 36 while moving. A unit of 20 Hellions? Yea, you can do that math.
Dark Light Technology
If you thought dark lance spam was gone, your fooling yourself. Dark Eldar are now able to field almost rediculas amounts of Blasters, Blast Pistols, Dark Lances, Heat Lances and the awe inspiring Void Lance. All of which are lance technology. I think the most popular unit in the Dark Eldar codex are trueborn. Able to carry up to 4 blasters and two heavy weapons (dark lance or splinter cannons). The new dark eldar can carry more dark light weaponry than the old.
I generally drive my trueborn right up into the center of the board (or sides if there is terrain up there), and dump them off into any area terrain. The Haemonculi with them will generate that Feel No Pain token you need, and the groups 4 blasters will take care of anything else. 4+ cover save, 4+ Feel No Pain, and your are delivering 4 blasters into the heart of the enemy. Fielding two of these units, with some close combat support nearby (incubi, talos, etc) and nothing is getting to your trueborn. You can also sit your trueborn back simply taking two dark lances with them.
Scourges get some of the cheapest Dark Lances in your army. Paying 15pts each for up to 4 in a 10 man unit. While Ravagers carry along 3 for an amazing 105pts. (screen these with raiders in the opening round and as much as you can during the game).
I have in several of my play lists, taken huge advantage of loading up with 20+ Dark Light weapons, and then taken tons of splinter cannons and other poisoned shooting weapons. This combined combo in your list does amazing things on the battlefield, and really pisses off your opponents. As an excercize, simply load up as many Dark Light Weapons as you can, and try and match it with your Splinter Cannons and Carbines. It gets ugly.
While all these are unique tactics to the Dark Eldar, these are also much more readily found and used. Next I will get back to the more obscure ones I have discovered through play.