In Burning Crusade, affliction instancing wasn't actually very dot heavy. It was pretty funny actually, I was throwing up two dots--Unstable Affliction and Corruption--on any boss, and only had to worry about keeping them refreshed while I spammed the shadow bolt button. It's no surprise really that once you had decent enough gear, the poor scaling of dots made destruction absolutely mandatory.
In WotLK, things have changed significantly for affliction. Not only have the previously lackluster dots had their damage significantly buffed in comparison to Corruption and UA--but a multitude of new buffs in the affliction tree which affect 'all your periodic effects' has made dot-locks far more dot-heavy than they were in the BC days. Not to mention about three hundred times more confusing! What's more, the introduction of Pandemic means that your dots scale not only with your spell damage, but with your crit rating as well, making it far less likely that destruction will prove to be the definitive best spec in higher tier content.
When I was first coming in to LK, I took my knowledge of BC raiding, and modified my rotation to fit my new best friend: Haunt. First, I'd open with Curse of Elements, and probably hit one of my spell damage trinkets. Then I'd begin my rotation with Haunt, follow it with UA, followed by corruption, and finally shadow bolt until either UA or Haunt needed to be refreshed. This actually worked exceedingly well, and I don't believe I actually felt the need to improve it until I had been doing heroics for awhile.
On the suggestion of a friend, I decided to try out some of the dots which I once scorned other warlocks for using. Siphon Life was the first addition to my rotation, after I noticed that I could get upwards of 500 damage per tick on a dot that didn't need to be refreshed very frequently at all. This gave me a nice little chunk of bonus damage. My rotation was modified again only recently, after I read the excellent post by fallenman on the warlock class forum. After reading it, I did some DPS tests on my own, played around with the numbers, tried to guess what I could expect to see in actual instances, and finally added immolate back into my primary rotation--a position it hasn't enjoyed since shortly after I made it through the dark portal I think.
At present, my casting rotation goes something like this:
[IMPORTANT NOTE: This rotation has been updated.]
0) If at any time, either of my two spell damage trinkets are up, I use them. They don't even have a global cooldown, so it shouldn't interrupt the rotation.
1) Get Curse of Elements on the target while you give the tank a moment to build a bit of threat.
2) Cast one Shadow Bolt, causing the Shadow Embrace effect.
3) Cast Haunt, causing both that debuff, and a second stack of the Shadow Embrace effect.
5) Siphon Life
6) Unstable Affliction
8) Re-apply dots as necessary, avoiding clipping (re-casting the dot before it has 100% completed.) Cast haunt whenever it's up--don't worry about clipping it (or corruption, the dot which it re-applies.) If not otherwise occupied, be casting Shadow Bolt.
The logic behind my casting order is thus: First, I get the Shadow Embrace and Haunt debuffs on the boss first thing, so that *each and every tick* of my dots gains the benefits of those effects. Corruption comes next because it is by far the most powerful dot in an affliction lock's arsenal, with UA being the closest second. Unstable Affliction and Immolate go together because they have very similar durations (thus allowing them to be re-cast together) and they go at the end of my initial rotation because their durations are very short, giving me the maximum amount of time to shadow bolt before I need to re-apply my dots.
Now in general, this is relatively close to what a lot of endgame raiders who play affliction do--which kinda surprises me, given that only recently did I read anything approaching an authoritative 'how to.' And that only led me to make one major change to the way I was doing things. However, there are some differences between what I do and 'what is best,' that I feel can be reasonably justified. Furthermore, there are a few things I've heard a large number of people insist upon which are frankly just dumb.
Perhaps the biggest divergence from the norm is the fact that I don't use Curse of Agony. It's not even easily accessable on my casting bar. At first I didn't use it because back in BC, using CoA was enough to get you booted from the raid if the leader knew even a little bit about warlocks. Then I started seeing everybody insist that in LK, it should be in your affliction rotation. I was skeptical, so I did tests. Curse of Elements came out on top for pure damage--which is something that all of the better affliction locks seem to agree upon. The reason that none of THEM use CoE is because--apparently--both boomkins and death knights have some kind of debuff that doesn't stack wich Curse of Elements, and is perhaps even better. I say apparently because I haven't grouped with any boomkin that I remember since level 70, and while I do know a DK who I group with often, I've yet to recieve a message saying 'a more powerful spell is already active' or something like that. Perhaps this is something I'll have to look into soon.
Another thing I've heard a great deal of is warlocks demanding that haunt be the LAST spell cast during the first rotation, rather than the first. The logic of this positively baffles me. Supposedly the idea is that you start with corruption, cast all your other dots, then cast haunt, refreshing the corruption that you started with, and giving you some time to shadow bolt before you have to recast haunt. I can't understand, however, how this minor benefit is supposed to make up for the roughly 1-2k damage more that your dots would have done if haunt had been active earlier. Not to mention shadow embrace! No no, I really don't see it.
Well, this old man has rambled long enough I suppose. So there you have it. My method, my results, and my defense of my results. All of this is of course, liable to change, because like anybody I am always trying to learn and improve. Sometimes my stubborn pride has a tendancy to stunt my ability to learn, (like hell will you ever see me cast Curse of Agony on a boss! >.>) but eventually I do.
I should perhaps note that there are a variety of topics which I touched on briefly here that require a bit more justification than I gave them. Said justification was excluded in favor of doing an entire future post on those topics, rather than cluttering up this one any further.