A common element of early D&D editions, the authors had an odd idea of balance. Instead of balancing level-by-level, the authors seemed to assume phases in the game where classes and concepts were most viable. For example, a Wizard is "officially" tier 1, but at level 1, physical characters (some Fighters and Monks, even!) can be very spiffy. After casting your 1-4 level 1 spells, what do you do to keep entertained?
Still, why is this notion (Rock Now, Suck Later OR Suck Now, Rock Later) so prevalent in 3.x? It seems to punish casters since so many campaigns end at level 6 or below. (Sucky prereqs are a common means of the Now & Later philosophy.) Those who rock later are repeatedly shut down by GMs who never run games at that level. The power of level 5 spells only matters if the group reaches that level.
How do y'all feel about this? Growing into and growing out of my usefulness by level seems... odd. Mind control is wonderful at low levels, but when the immunities hit, I'm stuck as something else, be it a new character or new facet of this character. It's like saying, "You must be in this level range to be viable with this concept," and since I like this so much, I'll officially coin it.
Now and Later Design: Abilities and concepts have a big, invisible (or nearly invisible) sign saying, "You must be in this level range to be viable." Outside this level range, you may as well play a different character, since your current one needs an overhaul. Enough optimization increases the viable level range.