I've played throughout the editions, and particularly 3rd edition throughout its run (with notably two campaigns that reached 16+ APL). I have never encountered the "CoDzilla" problem people keep complaining about.
I am not saying it isn't an issue, or that it hasn't been seen in action at some tables. Just that I have never actually experienced that problem myself. This leads me to believe this is (1) more a problem of players shooting deliberately for builds that milk some loophole or other (i.e. bad, counter-productive players, players who should be pointed at by the other players at the table as people who wreck everyone's fun because of a selfish agenda) rather than a true systematic problem running throughout the rules system itself, and (2) something that has been erected into a huge issue on message boards by
armchair theorists reasoning in terms of Spherical Cows and the like.
Yeah, except that the only way you haven't encounter the CoDzilla problem is if your druids were stupid.
Were they casting Summon Nature's Ally *? Congratulations. Those summons can grapple and attack, which quickly shuts down most enemies cold. With their feats, they could easily get +4 STR/+4 CON (a simple feat listed in the Player's Handbook that was clearly made for druids to take) and then those summons could become grapple/trip monsters. A Dire Wolf made trip attacks with every successful hit, with a bonus from size category, and an enormous strength. Resisting was nearly impossible for any humanoid.
Is it 'exploiting loopholes' to use a feat printed in the Player's Handbook for your class along with your iconic line of spells? Apparently yes.
How about Wildshape? You'd be an idiot not to take Natural Spell (A feat printed in the Player's Handbook that only your class could take) and that gave you access to all the forms in the monster manual?
How about your AC? Well, if you read the Dungeon Master's Guide (one of the three core books) there was a Wild Enchantment for armor that made the armor work in wildshape form, with bonuses (you ignored all armor penalties) and a type of plate armor clearly designed for you.
Did it take 'exploiting loopholes' to buy the armor designed for you to wear and wear it? Apparently it did.
And spells. Hah. Even the Spell Compendium had a number of totally busted options. Blinding Spittle ho.
If you never encountered the Druid outshining the rest of your players, it's probably because your druids were all played by the "Girlfriend of another player." And even THOSE have been known to solve entire realms of encounters just by spamming Summon Nature's Ally, with no real thought into best summons or anything else. After all, when you control 8 attackers you're just busy outshining the monk no matter what he's doing.
I'm going out on a limb here and saying you just haven't played much 3E. Because I played a LOT of 3E, and the optimization paths for the druid to break the game were all laid out, clear as day, as apparent and obvious as 'the fighter should get magical weapons' or 'monks should probably take those bracers that give them an armor class and were clearly designed to be taken by monks.' Druids were breaking the game from pretty much day 1.