I run 6 different mixed teams, 5 of them level 80 and one team is still level 71. The only classes not represented are DKs and rogues and none of my teams have less than four classes represented.

Having recently spent a while thinking about what I like the best, I would definitely go with:
1) Pally tank
2) Resto shammy or Holy priest healer (both are great, though tank heals are slightly easier with the shammy while party heals slightly easier with the priest)
3) Ele shammy (best off-healer, and for some of the hardest content when your gear is low, a 2nd toon doing a bit of healing is vital)
4) Warlock (best DPS I've found from any class, tons of flexibility)

The last DPS is up for debate, but I'd recommend:
- a hunter for good DPS and much easier gearing up (won't compete for drops with priests, locks, or shammies), or
- a mage for easy DPS, easier AOE, portals

Either the hunter (survival) or mage (frost) give replinishment as well.

One other option you could consider: replace the lock with a druid (maintaining the uber +13% spell damage buff) for the awesome +hit buff boomkins give, but that only helps if you are raiding with your team and the boomkin would do substantially less DPS than the lock.

I would highly encourage you to level mixed classes. With /click macros they are very easy to manage, are tons easier to gear up, give you more buffs, and give you lots of flexibility when you hit 80.