I personally prefer mixing my duos/trios up so I have more than 1 class lvl'd up, there's no rush in levelling, and I get to learn to play 2 classes at once. Try and vary which one you use as "main" and follower now n then too, if your setup/UI allows it, helps enjoy it more, and you learn more about playing both classes.

fyi, I'm talking PVE only in this post...

Rogues, Warriors and Warlocks are amazing with a healer as the only thing that causes them downtime is low health, make sure the healer on follow has plenty of regen, have them throw some dots/shocks/auto attack/wand a little and heal the main, and you can grind forever!

Any class will benefit like this from a healer class though, means soloing elite quests alot easier...

Combos I've tried that work for me so far:

-Warrior+Priest (any healer rocks though...)
-Hunter+Shadow Priest (lvl 30 so far) leave him in normal form, throw some heal over times (hots) and dots on the mob
-Lock+priest (as warrior)

I want to lvl:

Druid+rogue: I'll use the druid as a balance spec, but in caster form to throw heals on the rogue, and help dps. Also the chance of stealth x 2 is too much
Warrior+pali
Mage+pali or priest for AoE grinding

I have a lvl 70 lock and a lvl 54 Shaman, not sure what else I want to do yet