I'm going to completely refute this thread by claiming that all spells are at their right level. Whether the details of the individual spells are balanced or not is another matter - I'd rather the spell be fixed for it's level than changed level.
The reason I think this is that spells of a certain level are expected to allow the caster to do certain things. 3rd level spells let you do damage to a group of creatures (lightning bolt, fireball, etc.) or let the caster or one person travel anywhere (fly, water breathing). So if you have an adventure for levels 5-7, expect the party to be able to do these things. 5th level spells let you save the life of a party member (raise dead, break enchantment) or transport the whole party (overland flight, airy water, plane shift).
My views on some of the spells discussed:
Ray of Enfeeblement: Stays good right into the high levels, and really hurts anybody using Power Attack. But it doesn't stack with itself.
Sleep: In earlier editions, when magic users had 1 or 2 spells at first level, a spell which would let you "win" a single encounter was quite appropriate. In 3e, the balance is about right. Raising the casting time was a mistake IMHO.
Invisibility: In a world where invisibility is expected, this shouldn't be a problem. Guards will immediately expect invisibility if they hear something they can't see. Glitterdust is a pretty common spell IMC.
Polar Ray: This spell is ridiculously powerful. You can attack a creature in combat without hitting your allies - and you probably will hit, no worries about evasion and high saves. At fifteenth level, you're doing an average of 52 damage, so causing a massive damage save and doing respectable damage even to something with Cold Resistance 30.
Don't get my started about clerics with the Cold domain, Divine Metamagic, Split Ray and Domain Spontaneity with this spell
