To expand on the point of the person above me, if you have 10 land in a 60 card deck, you will expect to pick up a land every 6 draws. So your first hand will have an average of 1 land in it, and you have around a 1 in 6 chance to draw another land your first draw. That's 2 lands. What about the next turn? You will almost positively miss land drops. I've had decks that had 23 land and I still get mana screwed.
On the other hand, it doesn't take much land to improve the situation. Going from 23 land to 25 made a world of difference on the above deck.