Well, seems the playtesters weren't optimizers.
That is so very true and so very telling in regards to the evolution of D&D from 3.0 to 3.5 and finally 4.0.
3.0, and to a lesser extent 3.5 because it is a derivative, presume balance based on an ideal of how the game
should be played. 3.0 kept much of the ideal that was present in 1e and 2e but provided mechanics to go beyond that ideal. I think that is where a lot of the problems arose with broken class builds that were perfectly legal and, as the OP suggests, an inherent imbalance in the classes.
If you look at Clerics, Druids, and Wizards from the original 3.0 rules there are no really glaring imbalances.
Clerics are more powerful than their counterparts of earlier editions simply by means of having two more levels of spells and domains with their attendant powers. Ultimately their spell selection is pretty fair. Mostly bolstering and defensive with the occasional big bang to use against opposed alignment enemies. There are the occasional spells that break that mold, Blade Barrier springs to mind, but for the most part the cleric fills the defender/healer role.
Druids were given a bigger bump in 3.0 to make the class more enticing. Their class abilities (wildshape not included) are very niche oriented but pretty much indomitable in that niche. The basic druid spells of 3.0 were definitely given more juice. We see a lot more offensive spells and spells that were once based on environmental conditions are a lot less restricted. Even wildshape was balanced at first glance. Turning into animals and elementals only goes so far. But that is only one layer.
Wizards, under 3.0, had a lot of juice. They had more spells, more effective spells, and more implied ability with bonus feats, which I believe were geared toward meta-magic over item creation. But that is only under the basic rules.
Playtesting
NEVER reveals all of the exploits. Exploits are something that arise over time. Playtesting is there, IMO, to see if the class at its most basic functions does its job and holds its own against other classes.
The biggest disparity in balance arises from add-on rules. Splatbooks. Those extra fancy feats that let you cast spells while wildshaped or use turn attempts to increase caster level or whatever are a big game breaker. There is the argument that Sorcerer/Wizards receive similar splat benefits but ultimately it becomes an arms race. And there is always that belief that pure arcane casters are powerful enough so their bennies shouldn't go over the top.
Referring to the original quote about playtesters not being optimizers:
I can't help but chuckle at that in regard to 4e. I distinctly remember someone from the current WotC design pool saying, essentially, D&D players will always look for the best way to optimize their characters and that is one of the principles of design of 4e. Under 4e no character, whatever the class, should be more proficient in their role than any other character. And I say good on them if they've managed to pull that off. To me it looks like the homogenization of character design but that is most definitely the view of a jaundiced eye.
I've used some pretty broad strokes here, I know, but the point I'm trying to make is that there was most definitely a particular eye for balance at the origins of 3.0. This was colored by the implied balance of the previous editions and pretty much torn to shreds by later revisions. If you want to play the game as is, I'd suggest balancing factors beyond those stated by the rules.
Clerics and Druids are both highly religious classes. Religion suggests an obeisance to higher powers and ideals. Make clerics go on holy quests. Make druids defend sacred groves. Tithe, TITHE,
TITHE. If you recall in 1e (maybe 2e as well?) druids actually had to defeat higher ranked druids to advance.
Obviously this isn't everyone's style of play. If you just want to strap on your holy symbol or your sickle and mistletoe and go kick ass then these might not be solutions and are instead fabricated barriers. But it is important to remember that the balance is sometimes found in the way the game is [eye-roll]supposed[/eye-roll] to be played.
Personally, I can only take so much balance. If I run a game where things start to favor one or two characters too much it's either time for a smack on the ass for said characters or perks for everybody else. But that is purely my style. I guess I've adapted what I've been given. Or maybe adapted to what I've been given.