Judging from all the discussion, the emails, and the general state of most forums, it's clear to us all that dark eldar are in your meta, pillaging your game.
Officially, you shouldn't have access to the codex until the 5th of november, but if you're not blessed with a GW nearby for perusal, you've simply downloaded scans or summaries of the book.
So yes, dark eldar.
The name of the game is flexibility and options, but the internet isn't seeing this - yet. Once the little warseers and dakkaits start posting lists that differ greatly from each other, yet are still just as powerful, they'll know, but that's all in the future.
For now, we have the lovely cries of 'cheese and b0rkenz lancespam,' 'omgwtf combat armies,' 'cheap skimmers omg,' 'poisoned weapons rape my marines and tyranids metametameta,' and the general whine over feel no pain.
Knee-jerk reactions by scrubs? Yes. Partially caused by forumitis? You got that right.
No, dark eldar won't destroy any metas or games, unless you're still stuck in 3rd or 4th edition.
Foot armies (orks, demons, grey knights, tactical spam, footdar, tau gunlines, necrons, etc.) are almost unplayable versus dark eldar.
Proper 5th edition armies have nothing out of the ordinary to fear, since they all stack up well against each other.
Yeah, dark eldar gets melta, all three kinds of templates, combat units, effective troops, mobility, flexible and powerful load-outs, transports, psyker defense (offense, really, but the end result's the same), and even some durability, but that's what a 5th edition book looks like.
Basically, old, bad, horrendous armies and builds are gonna go extinct, while everything that works within the 5th edition environment's not likely to notice much difference.
The one and only change is the serious and very real risk of taking small heavy weapon units (long fangs, red devastators, hive guard, etc.), because dark eldar will put 12, 24, or even 36 shots at them the first turn with their venoms.
Here's a very brief list of what's really changing for the different armies...
Black templars: unaffected.Darklight doesn't work all that well against quad 'raiders immune to lance. Otherwise unaffacted. Proper templars aren't exactly good, but still largely competitive. Verdict: wants a new book, but can wait. Almost as unaffected as vanilla marines.
Chaos marines: endangered since 2008. Tri 'raider and 'classic' build now extinct.
Were good for all of three months. Dark eldar didn't change chaos' chances much, really, except for killing the tiny 'MSU' lash spam list, and tri 'raider. Verdict: wants a new codex, but since there are no players left, they can wait.
Demons: extinct.
Slow foot versus silly fast MSU mech equals almost guaranteed losses. Crippled by lack of own shooting. Verdict: needs a new codex not written by a fantasy player.
Orks: foot horde extinct. Hybrid endangered.When you can put down 90+ poisoned shots from cruise speed vehicles alone, the orks weep. Add to this the long range, high mobility, and outright better combat units, and the boyz in green aren't looking so good. Lootas are gonna be removed first turn, always. Verdict: needs a new codex. Likely to whine for once.
Eldar: endangered.
Mono builds of awfulness. If you tell a craftworlder that dark eldar do everything eldar does, only better, he or she's likely to agree with you. Dark eldar just brings more of everything, leaving eldar outgunned, outnumbered, and outmatched. What used to separate them (melta) is no more. Verdict: really needs a codex that's not a copy-paste from 3rd edition. Players will whine and defect to the dark side, so they can wait a bit.
Necrons: completely extinct.Dark eldar's speed, dakka, everpresent darklight, and powerful combat units rendered destroyer wing impotent and useless. Verdict: second most in need of a new codex.
Grey knights: completely extinct.
Dark eldar did what tyranids couldn't: destroy the quad raider list. MSU terminator squads die fast to venoms, and potentially losing squads of terminators to the crucible isn't appealing in the least. Dark eldar were the final nail in the coffin. Verdict: the faction most desperately in need of a new codex.
Tyranids: warseer lists extinct. Otherwise unaffected.Classic 'two monsters, three-thousand critters' now unplayable. Too much dakka, way too early in the game. Hive guard really hates venoms. Verdict: hive guard are range 18, and risk quick deaths to dakka. Little else's changed. Expect serious amounts of whining, and an even bigger refusal to change for 5th edition.
Sisters of battle: unaffected.
Still brings all the tools needed to beat face in 5th edition. Main change is exploding raiders and ravagers will kill some sisters every now and then. Verdict: little's changed. Carry on.
Imperial guard: foot guard extinct. Hybrid and everything else unaffected.
Foot guard's suffered the same fate as gunline tau and horde orks. Heavy weapon squads will go poof instantly, like every other tiny unit, ever. Good no one's using them. Since so few player run foot guard, things haven't really changed. Verdict: as good as pre-5th november.
Wolves: in general unaffected. Long fangs are an endangered species.
Thunderwolves need to be played more aggressively, or risk poofing to instant-slaughter attacks. Long fangs really don't like sucking down 12x'however many venoms are in range'-shots the first turn. Verdict: likely to start using armor over long fangs, or putting the 'fangs in rhinos.
Vampires: nipples endangered. Otherwise unaffected.Darklight killed the nipplestar. Devastators suffer at the hands of the venom, but less so than others, thanks to feel no pain from nearby priests. Verdict: Dante's golden boys are fairly impotent versus all the darklight. That's about it.
Vanilla marines: tactical spam extinct. Everything else unaffected.
Never used devastators to begin with. Gimmicky lists still work 'properly,' but foot marines are almost unplayable. Verdict: the least affected book.
Nerf angels: deathwing and doublewing endangered. Greenwing on foot extinct.
Tiny armies of land raiders, terminators, and overpriced bikes don't fare well against either darklight, 10+ combat units, or dakka. Verdict: just play them as regular marines already. Expect lots of whining.
Tau: proper lists unaffected. Gunlines extinct.
Lining up 60+ firewarriors and broadsides leads to turn 2 loss at best, or turn 1 loss at worst. The only concern for proper list is broadside-hammerhead balance, with more leaning towards hammerheads now. Verdict: gunline finally rendered unplayable. Expect lots of whining.
As you can see, 3rd and 4th edition are now over. If you want to keep playing gimped lists, feel free, but don't expect wins, challenge, or entertainment.
Don't be surprised if actual people outside the internet tell you to get a modern army, or they'll simply play someone else. You know, like what happened when 3rd edition officially became 4th, and everybody raped gunlines all over the place with combat armies.
The times, they are a-changing, but only for those stuck in the past.
