Ding, 58! You're officially eligible for Outland content now. Go ahead, step through the Dark Portal. The quality of Outland gear simply outclasses the stuff you will get in the old world, for the same time investment. Yes, Outland mobs will hit harder and fights will last longer, but the increased difficulty shouldn't faze a Warlock, and will certainly be pie if you brought a friend (preferably a healer) along. Be cautious with the pulls, remember your lessons, and you should do just fine in the Outland at level 58.
Completists among us would sneer at this suggestion to skip content. If you're in no hurry to hit 70, then more questing power to you!
The last stage of leveling from 61 to 70 in a nutshell: no new pets, no new mechanics. If you are level 60, you have pretty much learnt all the tricks of the Warlock trade. A 70 lock is essentially a 60 with (much) bigger numbers and some nifty tools to make this stage of the game less laborious.
For 61 levels you've been buffing yourself with Demon Armor/Skin - a nice but rather 'blah' self-buff. At level 62, the game gives you +50 spell damage with Fel Armor. Yes, spell damage - one of the most important stats for a DPS caster class. Rank 2 of Fel Armor, learnt at level 69, gives you a sweet 100 points of spell damage!
Fel Armor blows Demon Armor out of the water, period. Drop the latter off your bars to free up a button slot. Never ever forget reapplying Fel Armor to yourself every half hour! Demonic Aegis in the demonology tree makes Fel Armor even more outrageous (thanks, Diabla).
One final nuke for warlocks at level 64. This spell is a mana-efficient, faster-casting replacement for shadow bolt while soloing or short fights. It's got a nice, 'crawling' fire animation too!
Interestingly according to these calculations, as a chaser for immolate, this nuke tops shadow bolt in DPS if you don't have any points in the destruction tree. With Bane and Improved Shadow Bolt factored in, shadow bolts still reign for sustained combat.
Unless you spec heavily into the destruction tree (Emberstorm and Shadow & Flame) and have good fire-biased gear, the general consensus is that shadow bolt will remain your staple nuke.
Soulshatter, learnt at level 66, gives you an aggro dump every five minutes. At the cost of one soul shard, you reduce your all-around threat by 50%. Obviously you're not going to use this ability much while you're soloing, but Soulshatter is essential when it comes to group work and raids. Be careful using this AoE effect around mobs that haven't been aggro'ed, Soulshatter will actually turn them onto you! (Tested this again, soulshatter doesn't aggro mobs - thanks, Atilim.)
Ritual of Souls
It used to be a pain handing out healthstones via the trade window, especially when you're the only Warlock in a big raid. At level 68, you can create an automatic healthstone dispenser at the cost of just one soul shard, with the help of two other teammates. Each soulwell is good for 10 healthstones. Tell your helpers to not fidget during the ritual, or it'll fail and you can only do this again after five minutes. This spell is such a good idea that I'm surprised that mages only got their own food/drink dispenser last patch.
Seed of Corruption
If you like to make things go boom, this is the spell for you at level 70. Seed of Corruption (SoC) is a combo DoT/AoE effect that replaces Corruption. In other words, SoC overwrites corruption and you can't have both on a single target. When the seeded target takes 1044 damage from any source, the seed explodes, causing glorious AoE damage within 15 yards (cue maniacal laughter).
Non-lethal AoE damage from other SoC explosions, however, will not trigger a detonation. In other words, the explosion from target A has to kill target B, before it triggers the detonation in target B. This prevents you from setting off a 'chain' of explosions on a bunch of healthy targets.
Note that a seed's explosion only affects enemies around the seeded target. If target A's seed explodes, only target B and C gets the AoE damage, not target A. Target A will only suffer the DoT component of SoC, if any before detonation. So you'll want to seed as many different targets as possible to maximize the effect of this spell. The AoE damage also passes through walls and floors (This has been fixed last patch). As with any other AoE effect, be wary of attracting lots of unwanted attention from mobs!
At this point, you should have developed a good picture of what each talent tree offers to make an informed decision. There is no single 'best leveling spec' at this stage, although I am inclined to say that the Affliction gives you the most bang point for point, with little to no 'filler' talents.
I'd even venture to say that Blizzard did a good job with our trees, differentiating them through playstyle, rather than viability. All three trees give you good DPS, and only really differ in forms of delivery. Warlocks truly have a choice, and your choice simply depends on your preferred playing style, if you're not a min-maxer.
A recap of pros and cons of each tree:
If you're curious, this was my build at level 70, which was good for solo farming (there was a lot of farming to do as a fresh 70) and delivering some respectable burst damage on trash mobs in instances. Some warlocks swear by a Felguard demonology build such as this for leveling post-60, due to the pet's excellent DPS. For all intents and purposes, there is no one perfect build for all situations. Picking one talent over another is a value choice, and that entirely depends on your experience and preference. Check out Chris' excellent discussion of a 43/0/18 build.
There you go, a full-fledged cultist of the dark arts! For some, 70 represents the end of a long road (at least until Wrath of the Lich King). For many, this is just the beginning of the 'endgame', whether it's raiding, PvP or both.