I can understand that. Basically, the rules of 3E really require that the team have at the very least a primary Divine caster such as a Cleric or Druid, and strongly recommends a Wizard (though a Sorcerer or Psion may suffice). This is not explicit, but your average group of players will have a very hard time going through the game using the rules as written without those specific classes. If the game is heavy on stealth and traps, than either a caster skilled with anti-trap utility spells or a Rogue is pretty necessary.
Really, the 3E designers have admitted several times that the game was basically entirely designed with a four person team of a Fighter, Rogue, Wizard, and Cleric in mind. For example, designers spoke about how the rules in Savage Species were only written and tested based on how monster PCs would work using those four classes, and they didn't really examine how a Druid or Ranger monster PC would play out (even though those classes probably make more sense for the average monster PC than Rogue or Wizard). They just didn't bother to tell anyone that in the game books themselves. This had some messy repercussions regarding the design of most of the later classes, in my opinion. It is because of the problems caused by 3E's party assumption that 4E developed explicit class roles, which ultimately allows more flexibility in the class composition of the party.
In short, where 3E basically forces you to have a Wizard and Cleric and assumes you also have a Rogue and Fighter, 4E only requires a Leader and Defender at minimum, with a balanced team of all four roles being recommended.
Of course, because of things like healing surges and rituals, a 4E party really doesn't need to have any class in particular, or even a particular role, in order to get by.
I'm not seeing this difference of guideline versus requirement between the editions.
The basic roles have not really changed between the editions, 4e is just explicit about labeling some roles.
In 3e if you don't have a cleric or druid as a healer then you have a paladin, ranger, bard, or UMD rogue with wands of cure light wounds with only minor healing in combat or you use potions and pull back after fights to heal up.
I've played in a group where as a ranger with a wand I was the only healer for multiple levels of play and it went fine.
In 4e if you don't have a leader you are limited to second wind in combat and then have to survive until out of combat. I like surges as they make wands unnecessary. They are an improvement in not needing a healer.
In 3e if you don't have a trapfinding rogue you use certain magics to get around traps (fly over pits, neutralize poison to get around poison, heal the damage taken, clerical find traps will not help you) or you take the effects of the traps and soldier on (the barbarian trap finder method).
In 4e if you don't have a rogue or someone with thievery do you have more options than in 3e?
You said defenders are required in 4e, but it feels just like 3e to me, heavy combatants are useful but if you don't have someone in that role it can work it will just be different style of combat engagement without a tough shield wall hacking into the opposition.
In 3e UMD allows use of low level utility magics, sorcerers can use arcane items to fill out their needs for spells they don't know, cleric and druid magic does many things a wizard's does (divination, buffing, combat magic, terrain control), archer characters can provide the ranged artillery. Winged boots replace the need for a fly spell with other magic items serving similar functions for most any character. Wizards are useful and powerful but I don't see them as required in 3e but only guidelines for them in 4e.
Isn't the joke that the optimal party in 3e that can handle anything is all clerics or druids?
I like the 4e DMG analysis of different party compositions based on different missing roles, but I think it applies to 3e as well.
Significant differences:
4e healing surges allow less need of a healer role.
4e thievery can be mastered with a single feat. In 3e trapfinding tough traps is limited to a Rogue class feature combined with skills.
4e anybody can do utility magic with a feat and appropriate skill. In 3e magic is fairly class specific with some overlap between spell lists plus at higher levels you can fake it with UMD skill, mitigated a little with easy multiclassing for weak magic use.