That's a good question. When thinking about competitive, you must ask in the context of the meta. Are you talking about competitive to your playgroup and local game store, or just in the game of Magic itself?
Both answers are the same. You want your deck to do things as efficiently as possible to stop the other decks. This is done by fixing your mana base to be the most accurate, putting in cards that do more for less, perhaps adding in fetch lands to thin your deck. It really depends, as a good competitive deck is not clearly defined.