It's a massive jump there, I know. Really, though, there aren't too many significant changes for warlocks for that 25-level gap. You'll pick up the Felhunter at level 30, which affliction should switch to immediately as their favored pet. Beyond that, you really only see the introduction of Banish. You do get some great defensive tools with Death Coil and Shadow Ward, but they probably won't see too much use while leveling.
Spec updates Destruction should pick up Backdraft as soon as they are able; it really is one of the most amazing talents for leveling that you will get. Tossing a point over into Shadowburn is probably a good idea too, but only because you'll be picking up Soul Leech soon and that will be your only means of proccing it for a large number of levels. This was around the time when I started needing to use Life Tap as destruction, but killing things off with Shadowburn removed that completely.
Once you pick up Burning Embers, you should switch back to using your Imp, though it really doesn't make a huge impact at this level range. At level 49, I'd favor Shadowfury over Fire and Brimstone. You don't have Chaos Bolt or Incinerate yet, and the increased crit chance for Conflag is rather meh, so might as well pick up another survivability tool.
Affliction will finally get the amazing Shadow Embrace, but unfortunately, they still have to deal with the horribly slow Shadow Bolt in order to stack it. In most situations, you won't really even get any stacks up on a target before it's dead, but the talent is still great for what it is. You will also be able to snag Soul Swap, which is a lot of fun. Until you actually hit 50, you'll really only use it to finish off a target and toss your DoTs onto a new target -- unless you want to drop Glyph of Fear for Glyph of Soul Swap once you can, which is totally valid.
The biggest talent for demonology is Hand of Gul'dan, which is fantastic both solo and in dungeons. While solo, you'll probably want to stick with using the Felguard and having him charge in first to snag initial aggro, if you plan on using HoG. This way, it should remain in the aura left behind for a little while and get the increased damage. It isn't really a great solo spell, though (at least not this early), but it does shine in group content.
Your second glyph, which comes at level 50, is a much easier choice than the first one. For destruction, going with Immolate is pretty much a given. Imp is good, but having the faster, stronger Conflag is golden, not to mention the pure update on Immolate itself, which is going to be a huge portion of your damage in groups.
Demonology is a rather unfortunate one. It doesn't really have a second prime glyph that's helpful at this level. You can choose to go with Lash of Pain or Felguard for the time being; LoP is technically better, but the Felguard has a lot of soloing utility that's fantastic and sucks to give up. The only other choice you really have is Corruption, which just isn't that good for leveling or just in general.
Affliction rather ends up in the same position as demonology. Really, you want Haunt as your next glyph, but you won't have the spell for quite a few levels still. Also similar to demonology, you can go with Corruption for now instead, or you can pick up Agony. Those are pretty much the only choices you have.
Where to go? This is around the time that quests start to break up and you have to make some choices. I would say to focus on the high-end location that is closest to where you end up. Usually this will mean going Swamp of Sorrows to Blasted Lands or Felwood to Winterspring. Tanaris to Un'goro then Silithus is a workable option as well, but I'm usually not much of a fan. Getting to Tanaris is a bit of a pain, in my opinion, especially if you don't quest in Thousand Needles.
On the Horde side, I usually go with the Winterspring route. With Felwood and Winterspring, you can easily go from low to mid 40s all the way to 58 and then hit Outland using those two zones, and they are both really close to Org, which makes hopping back to the city for training, quests, or trading far easier and faster than the other options.
For the Alliance, I more so go with Searing Gorge, Badlands, Swamp of Sorrows and then Blasted Lands.
The end of the "original" content. This last little push holds quite a lot of goodies for warlocks. You'll finally get access to both of you secondary demons within this time span, which is just awesome -- who doesn't love dropping Infernals from the sky? You will also pick up Nethermancy, which is a flat 10% increase in your intellect. Pretty good deal just for leveling up. You also end up with Curse of the Elements and Soul Fire, finally giving a good use for Soulburn other than switching around pets.
Specs There aren't too many points to spend in this gap, but there are a few things to think about. Affliction has to choose between Pandemic and Everlasting Affliction at the start, and I would easily favor the former for leveling. You'll cast your curses and banes much more frequently, and you don't have Haunt to help with the reset on Corruption -- plus most things won't live through a single cast, anyway.
For the same reasons as above, if you skipped over Decimation previously in your demonology build, you should really pick it up before getting Cremation. You won't come across too much AOE while leveling where the Hellfire damage would help, and the Immolate reset isn't too grand. You will, however, be hitting a lot of low-health targets, which makes for a lot of tasty Soul Fire casting.
On the destruction side, favor Empowered Imp over picking up Bane of Havoc. I honestly can't think of any time that I used BoH while leveling until I hit Northrend because there simply isn't much of a opportunity to do so. Empowered Imp actually gives something useful, albeit highly annoying while leveling, due to the short duration on the proc.
Where to go? As I had mentioned in the previous section, Silithus, Wintersprings, or Blasted Lands are really the three primary zones that will get you to 58 and open up access to Outland. Winterspring can seem a bit dry on the quests at the very start, but after the first three one-offs, you get totally bombarded by them -- and all of them are kill quests, which is why I really love the zone. Why yes sir, I would love to go farm XP in order to get more XP. Thank you!
Blood Pact is a weekly column detailing DoTs, demons and all the dastardly deeds done by warlocks. We'll coach you in the fine art of staying alive, help pick the best target for Dark Intent, and steer you through encounters such as Blackwing Descent and The Bastion of Twilight.