I agree, Spec Affliction from 40-50 . Just Dot Dot Dot, Fear & let them die while you Dot another Mob... You'll 'blaze' through the levels.
As for Destro, with the addition of Burning crusade & the Auction house 'Outlands' gear. You can approach 25% crit rate with a full set of gear that costs 30-40g. Thats 25% Base, + 8% from Backlash & Ruin. Thats 33% crit rate. 1 out of every 3 spells... Not bad! Especially if your Shadowfurying 10-12 Horde at once, you'll see alot of '1200's' fly through the air. It's quite entertaining.
People will say, As a destruction warlock, you are a 'glass cannon' . & If your wearing your crit gear, its true you'll have far fewer hp's ( I have about 4k with my crit gear on ) But I only wear my crit gear on Large melee battlegrounds ( & instances ) where I know I'll be AOE'ing Madly & staying behind the safety of my allies.. In Smaller encounters, or when I'm leading or guarding a graveyard, I keep my Hitpoint gear on.
& there's nothing 'Glass' about 9k Hitpoints & a 2k HP Voidwalker shield ( lvl 60 ). But the cannon still applies, because I still have my Big damage, & about 15% crit rate.
Destruction is a more versatile build than you hear. The Survivability is great. Shadowfury will buy you 2 seconds to cast fear or Howl of terror Time & time again, & it can also stop a dozen or more horde in their tracks. The damage it does is just icing on the cake, but it I see people take a dirt nap from its crits all the time.
Voidwalker Bubble, Shadowfury + Howl of terror FTW! Time & time again...