Have had a request for some more information on forming a 10man group. This is a much bigger topic than 25s as 25s can be much more generic and are a lot more flexible. 10s you dont get anywhere near as much wiggle room to set up your group.
So I will start with a Naxx team as that is what most people will be working on right now.
Very simply you need the following:
You are not taking a MT and and OT. You are taking 2 tanks. Preferably one should be specced into AoE tanking, and one should be specced into single target. Both will be OTing at some stage and both will be MTing. They are of equal importance, DO NOT get the idea that one is a MT and more important than the other - therein lies the way of the elitist raider!
There is no good or bad combination of classes, only good and bad combination of specs and gear level. My favourite combo is Feral+Pally.
if you are taking 2 healers you need at least one direct healer and one AoE healer. A good combo is pally/Disc priest/shammy + Drood/Holy Priest/shammy. If you are taking 3 (recommended for newer groups to naxx10) then take 2 AoE type healers. My fave combo for healers is Pally (because I am one) Holy priest, druid.
Dee Pee Ess:
A even balance of melee and ranged is the go here, if possible. However, there is no encounter that REQUIRES a certain balance. You can clear Naxx with only melee or only ranged (but it is harder; KT and sapph will be very hard with all melee). But you are gimping your raid DPS if you do that buff wise. I reccomend 2 melee and 3 ranged. My fave combo is: ret pally, DK, huntard, mage, lock. But really, anything that covers off mana regen, melee buffs or anything like that will be good.
Speaking of covering off buffs with your group MMO-champion have a webpage that really helps with this. You can find it here
This is one combination I am fond of.
Here is another with 3 healers.