While I never really saw the CoDzilla in play due to mature players in my group (mature mindset, not age), and never letting them pull the 15 minute adventure day thing, I'm not so naive as to think it wasn't easily possible back in the day.
Edit: Sorry for the length. tl;dr - just check the "Pathfinder's Solution" paragraphs.
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What made Druidzilla work, as I recall was the following combinations:
1. Wildshape replaced ability scores. In a game with point-buy, especially when starting a character at higher levels (bypassing the suck levels prior to wildshape), you could pump up mental stats and spend your combat time in long term massive physical ability scores.
2. The Natural Spell feat. It allowed #1 to occur with full spellcasting. Hours per day (pretty much all day fairly quickly).
Also, at higher levels with Tiny wildshape and indirect spells like call lightning.
Nobody suspects the butterfly... muahahaha. (Okay, the tiny owl sitting in the tree there, then...)
3. Built in restrictions being whittled away with source material. Aside from the natural spell feat there were some pretty broken spells released later, making the "weaker spellcasting" not so weak anymore. That and the wildclasp armor stuff too (can't wear armor while wildshaped? now you can!). Can't wear heavier armor? Wood and Dragonscales it is!
It's felt like the right and not knowing what the left was doing. One group build a class with a bunch of powerful ability combinations, and weaken some aspects to cover this boost over other spellcasters... and then another group coming in and systematically stripping away these counterbalances.
Pathfinder's Solution
It was decided to adjust from the core of the class than to pick through all the little niggling things, so they went with toning back the main problem: wildshape, and it's really helped limit the "all around wonder" the druid once was. He can still do a lot, but has to build the character from the ground up in that way, sacrificing
something to be good at a concept.
Most of the broken spells weren't carried over either (not OGC anyways).
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The Clericzilla was not something I ran into often, but here's what I remember (anyone correct me if I'm wrong/forgetting something):
1. Heavy armor proficiency with 3/4 BAB, decent HD, and good saves, along with a really good set of buffing spells allowed him to be a decent Fighter in a pinch.
2. Spell options, such as find the path, commune, find traps, etc, all allowed them to get through a number of things normally requiring skilled characters.
Plus, while damage spells weren't their big thing, they did have access to a nice set of save or suck spells.
3. Domains gave some really superb abilities that basically gave them whole new roles (such as the Magic domain granting access to the utility of arcane spells in items, etc).
4. Put all those together and you get a class that can be built to fulfill nearly any role, fit any situation.
When you can seriously consider making 4 Clerics and doing as good (or better) than a normal spread of classes, you know the versatility might be a bit much.
If a single class can say "give me 24 hours rest, and I can come up with a solution" to practically any encounter or role requirement needed, you know something is up.
Now, personally, this didn't happen often for me because my players tended to play clerics as buffing and protecting others, rather than forcing the party to make tactics around spending the first 3-4 rounds of combat buffing up (that delay was felt much more significantly in practice than in theory), and doing these things hit hard on the 15 minute adventuring day aspect.
Though, Divine metamagic did take this to the level of absurdity.
Pathfinder's Solution
No more heavy armor proficiency (need to give up a feat or caster level to get it typically), the buffing spells were tweaked a bit (and without divine metamagic, there's a real damper on that tactic), the utility spells were changed to no longer be the auto-solutions they once were (this was partially done in 3.5e though, to be fair), and the Domains were toned back... nice options still, but not "use arcane items like a wizard" type of role-opening stuff.
Also, much of the new stuff the Cleric got was fairly "party centric" in boosting, rather than self interest. Channeling helps with the whole "all I cast is cure spells" while not giving too much power to the cleric, plus an added element of danger in it's use (healing enemies with your allies, even with the Selective feat).
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I still think the classes are fairly powerful, and may still have some options that are a little too good (an evil cleric's channeling might become an issue for a GM not ready to handle it, for example), however I feel that they've somehow made the classes more fun at the same time that they toned them back.
It's a rare thing to see a "nerf" so well accepted.
Ultimately, I feel they are classes that should be considered the "yardstick" to the level of versatility and power a class should have, but I don't think they ca break the game as badly as they did before.
And since anyone I play with tends to play them maturely anyways, it's been a non-issue for me.
