The D&D wizard spell memorization/preparation system is bad because it is extremely inflexible on a moment to moment basis. It often trades off frequency of use for power. E.g., a magic missile only does 1d4+1 damage per shot, but it is unerring and has a long range. Sleep has the potential to affect a number of creatures, but many creatures have no resistance to it. Spells take an entire slot, which for wizards, must be prepared ahead of time, but the effects are often powerful. This gives the low-level wizard effectively one, two, or three chances to make a difference with their magic. At all other times, they are waiting around to get their spells back, running in terror, or making feeble attacks with their melee/ranged weapons.
Compared to a system like the one found in Ars Magica -- well, there is no comparison. The flexibility of an Ars Magica system could be mated to a low power effect pool with more frequent hits on the caster's fatigue. It effectively becomes a "mana-based" system where minor spells are free (but not very powerful) and more significant spells weaken the caster.