I made a thread about this over on 339, and in that thread I pointed out that a well-rounded 3.5 party doesn't conform to the standard "roles" that have been historically pushed in previous D&D editions (Fighter, Cleric, Thief, and Magic-User) and that have been implemented in the new 4e system (Defender, Leader, Striker, and Controller). This is evident is that both Clerics and Druids technically fill the "divine" role suggested, but in reality are very different characters. Instead, a truly well-rounded party covers about 9 skill sets that sub-divide the roles, as follows:
Combat-oriented:
Defender - Like in 4e, this skill set includes the ability to keep the enemies away from the less durable members of the party. Whether that means they're taking the damage themselves or just negating the damage altogether is meaningless.
Controller - Actually encompasses both the Controller and much of the Leader aspects in 4e D&D. This character is responsible for buffing allies, controlling the battlefield, and occasionally debuffing enemies as needed.
Striker - Just the same as 4e, if nobody can deal damage to even a crippled enemy then you just can't win an encounter. Having characters capable of high damage output greatly decrease the resource drain on a party.
Skill/Magic-oriented:
Healer - Unlike in 4e, most characters can't heal themselves. Further, very few characters can actually heal the full range of afflictions. Sure a Rogue can use UMD to bring everyone's HP back up to snuff with a Wand of Cure Light Wounds, but if someone gets Diseased or Cursed (or even killed) then a lot of time and/or money is going to get dropped on removing them, making a character with natural healing abilities a lot more desirable.
Scout - This actually covers everything from opening a door and peeking inside to the fabled Scry and Die tactics of Wizard Assassins. Someone capable of seeing without being seen provides very valuable information to the party and can control the risks inherent in combat.
Traveler - This role runs into both Scout and Trapspringer to a high degree, but really has it's own distinctive skill sets. This role is all about getting the party where it needs to go. The traveler is the one at the top of a climbing team hammering in Pitons for the rest of the party to climb up, or going across a narrow ledge first and throwing back a rope so that the party has some extra stability for their own trip across, or opening a locked door such that the party can go through, or basically doing any other task to make the rest of the party have an easier time getting past hazards or obstacles.
Trapspringer - Whether it's a character that can actually spring traps without getting hurt or one that disables them, just about every dungeon delve needs a trapspringer to take care of this common, out-of-combat drain on resources.
Face - Someone capable of handling NPCs such that the party gets favorable rewards for their time or valuable information to complete their tasks. Absolutely critical for every party.
Encyclopaedia - Every party needs a brain, and for this specific role I'm essentially talking about the Decipher Script, Knowledge, and, to some extent, Spellcraft skills. This role compliments the Face's capabilities at gathering common information that is relevant for the party's specific situation while the Knowledge skills give more broadly applicable, academic knowledge.
There are a few skills that do not fall into any of these roles. These are Appraise, Concentration, Craft, Escape Artist, Forgery, Handle Animal, Perform, Profession, Ride, and Sleight of Hand. Generally speaking, these skills are all but completely unnecessary for the typical adventuring party, despite their usefulness to specific character builds (such as Concentration for spellcasters).
