Skill//(Experience) > Spec > Gear:
Having a knowledgeable MT and OT are key grips to any fight. You can have a MT do perfectly fine: for say, Kel'Thuzad (reference just for example), but your OT doesn't pick up the adds in the later phase of the fight, causing them to run loose throughout the raid.
Other than that, I'd have to say your skill and experience are the main things, if a tank has been raiding with a guild for a year, he/she is going to be the MT by right of passage. The guild feels more comfortable with him/her and will likely choose you as the OT. Knowing your rotation and cooldowns plays in to this also.